2007 EQUITY INCENTIVE PLAN OF PHARMASSET, INC.

Exhibit 4

2007 EQUITY INCENTIVE PLAN

OF

PHARMASSET, INC.

As amended through September 23, 2009

The purpose of this 2007 Equity Incentive Plan is to enhance the long-term stockholder value of Pharmasset, Inc. by offering opportunities to eligible individuals to participate in the growth in value of the equity of Pharmasset, Inc.

2.1. Definitions .

This Plan uses the following defined terms:

(a) “Administrator” means the Board or the Committee, or any officer or employee of the Company to whom the Board or the Committee delegates authority to administer this Plan.

(b) “Affiliate” means a “parent” or “subsidiary” (as each is defined in Section 424 of the Code) of the Company and any other entity that the Board or Committee designates as an “Affiliate” for purposes of this Plan.

(c) “Applicable Law” means any and all laws of whatever jurisdiction, within or without the United States, and the rules of any stock exchange or quotation system on which Shares are listed or quoted, applicable to the taking or refraining from taking of any action under this Plan, including the administration of this Plan and the issuance or transfer of Awards or Award Shares.

(d) “Award” means a Stock Award, SAR, Cash Award, or Option granted in accordance with the terms of this Plan.

(e) “Award Agreement” means the document evidencing the grant of an Award.

(f) “Award Shares” means Shares covered by an outstanding Award or purchased under an Award.

(g) “Awardee” means: (i) a person to whom an Award has been granted, including a holder of a Substitute Award or (ii) a person to whom an Award has been transferred in accordance with all applicable requirements of Sections 6.5, 7(h), and 17.

(h) “Board” means the Board of Directors of the Company.

(i) “Cash Award” means the right to receive cash as described in Section 8.3.

(j) “Cause” means employment related dishonesty, fraud, misconduct or disclosure or misuse of confidential information, or other employment related conduct that is likely to cause significant injury to the Company, an Affiliate, or any of their respective employees, officers or directors (including, without limitation, commission of a felony or similar offense), in each case as determined by the Administrator. “Cause” shall not require that a civil judgment or criminal conviction have been entered against or guilty plea shall have been made by the Awardee regarding any of the matters referred to in the previous sentence. Accordingly, the Administrator shall be entitled to determine “Cause” based on the Administrator’s good faith belief. If the Awardee is criminally charged with a felony or similar offense that shall be a sufficient, but not a necessary, basis for such belief.

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(k) “Change in Control” means any transaction or event that the Board specifies as a Change in Control under Section 10.4.

(l) “Code” means the Internal Revenue Code of 1986.

(m) “Committee” means a committee composed of Company Directors appointed in accordance with the Company’s charter documents and Section 4.

(n) “Company” means Pharmasset, Inc., a Delaware corporation.

(o) “Company Director” means a member of the Board.

(p) “Consultant” means an individual who, or an employee or agent of any entity that, provides bona fide services to the Company or an Affiliate not in connection with the offer or sale of securities in a capital-raising transaction, but who is not an Employee.

(q) “Director” means a member of the Board of Directors of the Company or an Affiliate.

(r) “Domestic Relations Order” means a “domestic relations order” as defined in, and otherwise meeting the requirements of, Section 414(p) of the Code, except that reference to a “plan” in that definition shall be to this Plan.

(s) “Effective Date” means the first date of the sale by the Company of shares of its capital stock in an initial public offering pursuant to a registration statement on Form S-1 filed with the SEC.

(t) “Employee” means a regular employee of the Company or an Affiliate, including an officer or Director, who is treated as an employee in the personnel records of the Company or an Affiliate, but not individuals who are classified by the Company or an Affiliate as: (i) leased from or otherwise employed by a third party, (ii) independent contractors, or (iii) intermittent or temporary workers. The Company’s or an Affiliate’s classification of an individual as an “Employee” (or as not an “Employee”) for purposes of this Plan shall not be altered retroactively even if that classification is changed retroactively for another purpose as a result of an audit, litigation or otherwise. An Awardee shall not cease to be an Employee due to transfers between locations of the Company, or between the Company and an Affiliate, or to any successor to the Company or an Affiliate that assumes the Awardee’s Options under Section 10. Neither service as a Director nor receipt of a director’s fee shall be sufficient to make a Director an “Employee.”

(u) “Exchange Act” means the Securities Exchange Act of 1934.

(v) “Executive” means, if the Company has any class of any equity security registered under Section 12 of the Exchange Act, an individual who is subject to Section 16 of the Exchange Act or who is a “covered employee” under Section 162(m) of the Code, in either case because of the individual’s relationship with the Company or an Affiliate. If the Company does not have any class of any equity security registered under Section 12 of the Exchange Act, “Executive” means any (i) Director, (ii) officer elected or appointed by the Board, or (iii) beneficial owner of more than 10% of any class of the Company’s equity securities.

(w) “Expiration Date” means, with respect to an Award, the date stated in the Award Agreement as the expiration date of the Award or, if no such date is stated in the Award Agreement, then the last day of the maximum exercise period for the Award, disregarding the effect of an Awardee’s Termination or any other event that would shorten that period.

(x) “Fair Market Value” means the value of Shares as determined under Section 18.2.

(y) “Fundamental Transaction” means any transaction or event described in Section 10.3.

(z) “Good Reason” means (i) a material diminution in responsibility or compensation, or (ii) requiring Awardee to work in a location (other than normal business travel) which is more than 50 miles from Awardee’s principal place of employment before the change.

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(aa) “Grant Date” means the date the Administrator approves the grant of an Award. However, if the Administrator specifies that an Award’s Grant Date is a future date or the date on which a condition is satisfied, the Grant Date for such Award is that future date or the date that the condition is satisfied.

(bb) “Incentive Stock Option” means an Option intended to qualify as an incentive stock option under Section 422 of the Code and designated as an Incentive Stock Option in the Award Agreement for that Option.

(cc) “Involuntary Termination” means Termination by the Company without Cause or Termination by the Awardee for Good Reason.

(dd) “Nasdaq” means the Nasdaq Global Market or its successor.

(ee) “Nonstatutory Option” means any Option other than an Incentive Stock Option.

(ff) “Objectively Determinable Performance Condition” shall mean a performance condition (i) that is established (A) at the time an Award is granted or (B) no later than the earlier of (1) 90 days after the beginning of the period of service to which it relates, or (2) before the elapse of 25% of the period of service to which it relates, (ii) that is uncertain of achievement at the time it is established, and (iii) the achievement of which is determinable by a third party with knowledge of the relevant facts. Examples of measures that may be used in Objectively Determinable Performance Conditions include net order dollars, net profit dollars, net profit growth, net revenue dollars, revenue growth, individual performance, earnings per share, return on assets, return on equity, and other financial objectives, objective customer satisfaction indicators and efficiency measures, each with respect to the Company and/or an Affiliate or individual business unit.

(gg) “Officer” means an officer of the Company as defined in Rule 16a-1 adopted under the Exchange Act.

(hh) “Option” means a right to purchase Shares of the Company granted under this Plan.

(ii) “Option Price” means the price payable under an Option for Shares, not including any amount payable in respect of withholding or other taxes.

(jj) “Option Shares” means Shares covered by an outstanding Option or purchased under an Option.

(kk) “Plan” means this 2007 Equity Incentive Plan of Pharmasset, Inc.

(ll) “Prior Plan” means the Company’s 1998 Stock Plan (as amended).

(mm) “Purchase Price” means the price payable under a Stock Award for Shares, not including any amount payable in respect of withholding or other taxes.

(nn) “Retirement” means Termination with the consent of the Company after the attainment of age 60 and the completion of five years of continuous service with the Company and its Affiliates (including any predecessor entities).

(oo) “Rule 16b-3” means Rule 16b-3 adopted under Section 16(b) of the Exchange Act.

(pp) “SAR” or “Stock Appreciation Right” means a right to receive cash and/or Shares based on a change in the Fair Market Value of a specific number of Shares pursuant to an Award Agreement, as described in Section 8.1.

(qq) “Securities Act” means the Securities Act of 1933.

(rr) “Share” means a share of the common stock of the Company or other securities substituted for the common stock under Section 10.

(ss) “Stock Award” means an offer by the Company to sell shares subject to certain restrictions pursuant to the Award Agreement as described in Section 8.2 or, as determined by the Committee, a

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notional account representing the right to be paid an amount based on Shares. Types of Awards which may be granted as Stock Awards include such awards as are commonly known as restricted stock, deferred stock, restricted stock units, performance shares, phantom stock or similar types of awards as determined by the Administrator.

(tt) “Substitute Award” means a Substitute Option, Substitute SAR or Substitute Stock Award granted in accordance with the terms of this Plan.

(uu) “Substitute Option” means an Option granted in substitution for, or upon the conversion of, an option granted by another entity to purchase equity securities in the granting entity.

(vv) “ Substitute SAR” means a SAR granted in substitution for, or upon the conversion of, a stock appreciation right granted by another entity with respect to equity securities in the granting entity.

(ww) “Substitute Stock Award” means a Stock Award granted in substitution for, or upon the conversion of, a stock award granted by another entity to purchase equity securities in the granting entity.

(xx) “Termination” means that the Awardee has ceased to be, with or without any cause or reason, an Employee, Director or Consultant. However, unless so determined by the Administrator, or otherwise provided in this Plan, “Termination” shall not include a change in status from an Employee, Consultant or Director to another such status. An event that causes an Affiliate to cease being an Affiliate shall be treated as the “Termination” of that Affiliate’s Employees, Directors, and Consultants.

2.2. Rules of Interpretation . Any reference to a “Section,” without more, is to a Section of this Plan. Captions and titles are used for convenience in this Plan and shall not, by themselves, determine the meaning of this Plan. Except when otherwise indicated by the context, the singular includes the plural and vice versa. Any reference to a statute is also a reference to the applicable rules and regulations adopted under that statute. Any reference to a statute, rule or regulation, or to a section of a statute, rule or regulation, is a reference to that statute, rule, regulation, or section as amended from time to time, both before and after the Effective Date and including any successor provisions.

3.1. Number of Award Shares .

(a) The Shares issuable under this Plan shall be authorized but unissued or reacquired Shares, including Shares repurchased by the Company on the open market. The number of Shares initially available for issuance over the term of this Plan shall be 1,371,190. The maximum number of Shares available for issuance hereunder shall be increased by the number of Shares subject to stock options issued under the Prior Plan that expire, terminate or are canceled or forfeited for any reason after the Effective Date without having been exercised in full. Subject to the approval of stockholders, the maximum number of Shares available for issuance hereunder was further increased by 1,000,000 by action of the Board dated July 15, 2009.

(b) If and to the extent that an Award granted under this Plan expires, terminates or is canceled or forfeited for any reason without the issuance of all the Shares subject thereto, those unissued Shares will again become available for grant under the Plan. Similarly, if any Shares issued pursuant to an Award granted under this Plan are forfeited or repurchased at the original purchase price or less for any reason, those Shares will again become available for grant under the Plan.

3.2. Source of Shares . Award Shares may be: (a) Shares that have never been issued, (b) Shares that have been issued but are no longer outstanding, or (c) Shares that are outstanding and are acquired to discharge the Company’s obligation to deliver Award Shares.

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3.3. Term of this Plan .

(a) This Plan shall be effective on the Effective Date, and Awards may be granted under this Plan on and after, the Effective Date. Upon effectiveness of this Plan, no additional awards will be made under the Prior Plan.

(b) Subject to the provisions of Section 14, Awards may be granted under this Plan for a period of ten years from the latest date the Company’s stockholders approve this Plan.

4.1. General .

(a) The Board shall have ultimate responsibility for administering this Plan. To the extent permitted by Applicable Law, the Board may delegate certain of its responsibilities to a Committee, which shall consist of at least two members of the Board. In addition, to the extent permitted by Applicable Law, the Board or the Committee may further delegate its responsibilities to any Employee of the Company or any Affiliate. Where this Plan specifies that an action is to be taken or a determination made by the Board, only the Board may take that action or make that determination. Where this Plan specifies that an action is to be taken or a determination made by the Committee, only the Committee may take that action or make that determination; provided that, if for some reason the Committee cannot act or make a determination, then the Board shall also be entitled to take such action or make such determination. Where this Plan references the “Administrator,” the action may be taken or determination made by the Board, the Committee, or other Administrator. However, only the Board or the Committee may approve grants of Awards to Executives or Non-Employee Directors, and an Administrator other than the Board or the Committee may grant Awards only within the guidelines established by the Board or Committee. Moreover, all actions and determinations by any Administrator are subject to the provisions of this Plan.

(b) So long as the Company has registered and outstanding a class of equity securities under Section 12 of the Exchange Act and to the extent necessary or helpful to comply with Applicable Law with respect to officers subject to Section 16 or the Exchange Act and/or others, the Committee shall consist of Company Directors who are “Non-Employee Directors” as defined in Rule 16b-3 and, after the expiration of any transition period permitted by Treasury Regulations Section 1.162-27(h)(3), who are “outside directors” as defined in Section 162(m) of the Code. So long as the Shares are listed with Nasdaq, the Committee shall comply with applicable Nasdaq rules and listing standards.

4.2. Authority of the Board or the Committee . Subject to the other provisions of this Plan, the Board or the Committee shall have the authority to:

(a) grant Awards, including Substitute Awards;

(b) determine the Fair Market Value of Shares;

(c) determine the Option Price and the Purchase Price of Awards;

(d) select the Awardees;

(e) determine the times Awards are granted;

(f) determine the number of Shares subject to each Award;

(g) determine the methods of payment that may be used to purchase Award Shares;

(h) determine the methods of payment that may be used to satisfy withholding tax obligations;

(i) determine the other terms of each Award, including but not limited to the time or times at which Awards may be exercised, whether and under what conditions an Award is assignable, whether an Option is a Nonstatutory Option or an Incentive Stock Option, automatic cancellation of the Award if certain objective requirements determined by the Administration are not met;

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(j) modify or amend any Award;

(k) authorize any person to sign any Award Agreement or other document related to this Plan on behalf of the Company;

(l) determine the form of any Award Agreement or other document related to this Plan, and whether that document, including signatures, may be in electronic form;

(m) interpret this Plan and any Award Agreement or document related to this Plan;

(n) correct any defect, remedy any omission, or reconcile any inconsistency in this Plan, any Award Agreement or any other document related to this Plan;

(o) adopt, amend, and revoke rules and regulations under this Plan, including rules and regulations relating to sub-plans and Plan addenda;

(p) adopt, amend, and revoke special rules and procedures which may be inconsistent with the terms of this Plan, set forth (if the Administrator so chooses) in sub-plans regarding (for example) the operation and administration of this Plan and the terms of Awards, if and to the extent necessary or useful to accommodate non-U.S. Applicable Laws and practices as they apply to Awards and Award Shares held by, or granted or issued to, persons working or resident outside of the United States or employed by Affiliates incorporated outside the United States;

(q) determine whether a transaction or event should be treated as a Change in Control;

(r) determine the effect of a Fundamental Transaction and, if the Board determines that a transaction or event should be treated as a Change in Control, then the effect of that Change in Control;

(s) appoint such additional administrators as are necessary to perform various administrative acts and determine the duties of such administrators; and

(t) make all other determinations the Administrator deems necessary or advisable for the administration of this Plan.

4.3. Scope of Discretion . Subject to the provisions of this Section 4.3, on all matters for which this Plan confers the authority, right or power on the Board, the Committee, or other Administrator to make decisions, that body may make those decisions in its sole and absolute discretion. Those decisions will be final, binding and conclusive. In making its decisions, the Board, Committee or other Administrator need not treat all persons eligible to receive Awards, all Awardees, all Awards or all Award Shares the same way. Notwithstanding anything herein to the contrary, and except as provided in Section 13.3, the discretion of the Board, Committee or other Administrator is subject to the specific provisions and specific limitations of this Plan, as well as all rights conferred on specific Awardees by Award Agreements and other agreements.

4.4. Vesting of Awards . Awards will be subject to such vesting or forfeiture conditions as the Administrator may determine, subject to the following guidelines:

(a) With respect to Awards that vest (or that are earned or become non-forfeitable) based, in whole or in part, on the achievement of one or more performance conditions (including but not limited to Objectively Determinable Performance Conditions), the period over which such performance is measured will be at least one year.

(b) Except as otherwise provided in Section 11.1, with respect to Awards that vest (or that are earned or become non-forfeitable) solely based on the service of the Awardee to the Company and its Affiliates, the requisite service period for the Award to become fully vested (or earned or non-forfeitable) will be at least three years (provided that the Award may vest ratably over that period).

(c) Notwithstanding the foregoing, or any other provision of this Plan:

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4.5. Repricing . Notwithstanding any other provision of this Plan, no Option or SAR may be repriced, directly or indirectly, without approval of the Company’s stockholders. For this purpose, the method of obtaining such stockholder approval and the required degree of approval will be determined in accordance with applicable state law and the Company’s governing documents.

5.1. Eligible Individuals . Awards (including Substitute Awards) may be granted to, and only to, Employees, Directors and Consultants, including to prospective Employees, Directors and Consultants conditioned on the beginning of their service for the Company or an Affiliate. However, Incentive Stock Options may only be granted to Employees, as provided in Section 7(g).

5.2. Section 162(m) Limitation .

(a) Options and SARs . Subject to the provisions of this Section 5.2, for so long as the Company is a “publicly held corporation” within the meaning of Section 162(m) of the Code: (i) no Employee may be granted one or more SARs or Options within any fiscal year of the Company under this Plan to purchase or be issued more than 1,000,000 Shares under Options or to receive compensation calculated with reference to more than that number of Shares under SARs, subject to adjustment pursuant to Section 10, and (ii) Options and SARs may be granted to an Executive only by the Committee (and, notwithstanding anything to the contrary in Section 4.1(a), not by the Board). If an Option or SAR is cancelled without being exercised or if the Option Price of an Option is reduced, that cancelled or repriced Option or SAR shall continue to be counted against the limit on Awards that may be granted to any individual under this Section 5.2.

(b) Cash Awards and Stock Awards . Any Cash Award or Stock Award intended as “qualified performance-based compensation” within the meaning of Section 162(m) of the Code must be awarded, vest or become exercisable contingent on the achievement of one or more Objectively Determinable Performance Conditions. The Committee shall have the discretion to determine the time and manner of compliance with Section 162(m) of the Code. The maximum annual value of Cash Awards or Stock Awards to any individual may not exceed $3,000,000.

The following rules apply to all Options:

6.1. Price . No Option may have an Option Price less than the Fair Market Value of the Shares on the Grant Date. In no event will the Option Price of any Option be less than the par value of the Shares issuable under the Option if that is required by Applicable Law. The Option Price of an Incentive Stock Option shall be subject to the additional requirement stated in Section 7(f). Notwithstanding the foregoing, in the event an Option is granted with an exercise price less than that set forth in this Section 6.1, if the mistake was unintentional, a violation of this provision shall not cause such Option to be void or voidable.

6.2. Term . No Option shall be exercisable after its Expiration Date. No Option may have an Expiration Date that is more than ten years after its Grant Date. Additional provisions regarding the term of Incentive Stock Options are provided in Sections 7(a) and 7(e).

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6.3. Vesting . Options shall be exercisable: (a) on the Grant Date, or (b) in accordance with a schedule related to the Grant Date, the date the Awardee’s directorship, employment or consultancy begins, or a different date specified in the Award Agreement. Additional provisions regarding the vesting of Incentive Stock Options are provided in Section 7(c). No Option granted to an individual who is subject to the overtime pay provisions of the Fair Labor Standards Act may be exercised before the expiration of six months after the Grant Date.

6.4. Form and Method of Payment .

(a) The Board or Committee shall determine the acceptable form and method of payment for exercising an Option. So long as variable accounting pursuant to “APB 25” does not apply and the Board or Committee otherwise determines there is no material adverse accounting consequence at the time of exercise, the Board or Committee may require the delivery in Shares for the value of the net appreciation of the Shares at the time of exercise over the exercise price. The difference between full number of Shares covered by the exercised portion of the Award and the number of Shares actually delivered shall be restored to the amount of Shares reserved for issuance under Section 3.1.

(b) Acceptable forms of payment for all Option Shares are cash, check or wire transfer, denominated in U.S. dollars except as specified by the Administrator for non-U.S. Employees or non-U.S. sub-plans.

(c) In addition, the Administrator may permit payment to be made by any of the following methods:

(i) other Shares, or the designation of other Shares, which (A) are “mature” shares for purposes of avoiding variable accounting treatment under generally accepted accounting principles (generally mature shares are those that have been owned by the Awardee for more than six months on the date of surrender), and (B) have a Fair Market Value on the date of surrender equal to the Option Price of the Shares as to which the Option is being exercised;

(ii) provided that a public market exists for the Shares, consideration received by the Company under a procedure under which a licensed broker-dealer advances funds on behalf of an Awardee or sells Option Shares on behalf of an Awardee (a “Cashless Exercise Procedure” ) , provided that if the Company extends or arranges for the extension of credit to an Awardee under any Cashless Exercise Procedure, no Officer or Director may participate in that Cashless Exercise Procedure;

(iii) cancellation of any debt owed by the Company or any Affiliate to the Awardee by the Company including without limitation waiver of compensation due or accrued for services previously rendered to the Company; and

(iv) any combination of the methods of payment permitted by any paragraph of this Section 6.4.

(d) The Administrator may also permit any other form or method of payment for Option Shares permitted by Applicable Law.

6.5. Nonassignability of Options . Except as determined by the Administrator, no Option shall be assignable or otherwise transferable by the Awardee except by will or by the laws of descent and distribution. However, Options may be transferred and exercised in accordance with a Domestic Relations Order and may be exercised by a guardian or conservator appointed to act for the Awardee. Incentive Stock Options may only be assigned in compliance with Section 7(h).

6.6. Substitute Options . The Board may cause the Company to grant Substitute Options in connection with the acquisition by the Company or an Affiliate of equity securities of any entity (including by merger, tender offer, or other similar transaction) or of all or a portion of the assets of any entity. Any such substitution shall be effective on the effective date of the acquisition. Substitute Options may be Nonstatutory Options or Incentive Stock Options. Unless and to the extent specified otherwise by the Board, Substitute Options shall have the same terms and conditions as the options they replace, except that (subject to the provisions of Section 10) Substitute Options shall be Options to purchase Shares rather than equity securities of the granting entity, shall have an Option Price determined by the Board and shall be on terms that, as determined by the Board in its sole and absolute discretion, properly reflect the substitution.

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The following rules apply only to Incentive Stock Options and only to the extent these rules are more restrictive than the rules that would otherwise apply under this Plan. With the consent of the Awardee, or where this Plan provides that an action may be taken notwithstanding any other provision of this Plan, the Administrator may deviate from the requirements of this Section, notwithstanding that any Incentive Stock Option modified by the Administrator will thereafter be treated as a Nonstatutory Option.

(a) The Expiration Date of an Incentive Stock Option shall not be later than ten years from its Grant Date, with the result that no Incentive Stock Option may be exercised after the expiration of ten years from its Grant Date.

(b) No Incentive Stock Option may be granted more than ten years from the date this Plan was approved by the Board.

(c) Options intended to be incentive stock options under Section 422 of the Code that are granted to any single Awardee under all incentive stock option plans of the Company and its Affiliates, including incentive stock options granted unde