FORM OF FIFTH SUPPLEMENTAL INDENTURE

Exhibit 4.1

FIFTH SUPPLEMENTAL INDENTURE

among

VECTREN UTILITY HOLDINGS, INC., AS ISSUER

INDIANA GAS COMPANY, INC., AS GUARANTOR

SOUTHERN INDIANA GAS AND ELECTRIC COMPANY, AS GUARANTOR

VECTREN ENERGY DELIVERY OF OHIO, INC., AS GUARANTOR

and

U.S. BANK NATIONAL ASSOCIATION, AS TRUSTEE

Dated October 18, 2006

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TABLE OF CONTENTS

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FIFTH SUPPLEMENTAL INDENTURE, dated as of October 18, 2006 (the “Fifth Supplemental Indenture”), among Vectren Utility Holdings, Inc., an Indiana corporation (the “Company”), Indiana Gas Company, Inc., an Indiana corporation and an Ohio corporation (“Indiana Gas”), Southern Indiana Gas and Electric Company, an Indiana corporation (“SIGECO”) and Vectren Energy Delivery of Ohio, Inc., an Ohio corporation (“VEDO”, and together with Indiana Gas and SIGECO, the “Initial Guarantors”) and U.S. Bank National Association (the “Trustee”).

WHEREAS, the Company and the Initial Guarantors executed and delivered the Indenture dated as of October 19, 2001 (the “Base Indenture”) to the Trustee to provide for the Company’s unsecured notes, debentures or other evidence of indebtedness of the Company (collectively, the “Securities”), and the Guarantees (as hereinafter defined), to be issued from time to time in one or more series, as might be determined by the Company under the Base Indenture;

WHEREAS, pursuant to the terms of the Base Indenture, the Company desires to provide for the establishment of a new series of Debt Securities, the 5.95% Insured Quarterly Notes due 2036 (the “IQ Notes”), and the unconditional guarantees by the Guarantors (as defined herein) of the payment of the amounts owed with respect to the IQ Notes (the “Guarantees”), the form and terms of such IQ Notes and the terms, provisions and conditions of the IQ Notes and the Guarantees to be set forth as provided in the Base Indenture and this Fifth Supplemental Indenture (together, the “Indenture”);

WHEREAS, the Company and the Initial Guarantors requested that the Trustee execute and deliver this Fifth Supplemental Indenture and all requirements necessary to make this Fifth Supplemental Indenture a valid, binding and enforceable instrument in accordance with its terms, and to make the IQ Notes, when executed, authenticated and delivered by the Company and with the Guarantees endorsed thereon and executed by the Guarantors, the valid, binding and enforceable obligations of the Company and the Guarantors, as applicable, have been made:

NOW, THEREFORE, in consideration of the purchase and acceptance of the IQ Notes by the Holders thereof, and for the purpose of setting forth, as provided in the Base Indenture, the form and terms of the IQ Notes, each of the Company and the Initial Guarantors, as applicable, covenants and agrees with the Trustee as follows:

ARTICLE I

DEFINITIONS

SECTION 1.1. Definition of Terms.

(a) Unless the context otherwise requires:

(b) a term defined in the Base Indenture has the same meaning when used in this Fifth Supplemental Indenture;

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(c) a term defined anywhere in this Fifth Supplemental Indenture has the same meaning throughout;

(d) the singular includes the plural and vice versa;

(e) headings are for convenience of reference only and do not affect interpretation;

(f) the following terms have the meanings given to them in this Section 1.1(f):

“Beneficial Owner” has the meaning specified in Section 3.4.

“Fiscal Agent” means U.S. Bank Trust National Association, New York, New York, or its successor.

“Global Note” shall have the meaning set forth in Section 2.4.

“Guarantors” shall have the meaning specified in Section 2.1.

“Initial Period” has the meaning specified in Section 3.4.

“Insurance Agreement” means that certain Insurance Agreement, dated as of October 18, 2006, by and between the Company and the Insurer.

“Insurer” means Financial Guaranty Insurance Company, a New York stock insurance corporation, or any successor thereto.

“Interest Payment Date” means January 1, April 1, July 1 and October 1 of each year, beginning January 1, 2007.

“Maturity Date” shall have the meaning specified in Section 2.2.

“Original Issue Date” means October 18, 2006.

“Participants” shall have the meaning specified in Section 3.4.

“Policy” shall mean the surety bond for the benefit of the Holders of the IQ Notes issued by the Insurer with respect to payments due for principal of and interest on the IQ Notes as provided in such surety bond.

“Redemption Price” shall have the meaning specified in Section 3.1.

“Redemption Request” shall have the meaning specified in Section 3.4.

“Regular Record Date” means, with respect to any Interest Payment Date for the IQ Notes, the close of business on the fifteenth day (whether or not a Business Day) prior to such Interest Payment Date.

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“Representatives” shall have the meaning specified in Section 3.4.

“Subsequent Period” shall have the meaning specified in Section 3.4.

The following terms shall have the respective meanings set forth in the recitals to this Fifth Supplemental Indenture:

“Base Indenture”

“Company”

“Guarantees”

“Indenture”

“Indiana Gas”

“Initial Guarantors”

“IQ Notes”

“Fifth Supplemental Indenture”

“Securities”

“SIGECO”

“Trustee”

“VEDO”

ARTICLE II

GENERAL TERMS AND CONDITIONS OF THE NOTES

SECTION 2.1. Designation and Principal Amount; Guarantees.

There is hereby authorized a new series of Securities, designated the 5.95% Insured Quarterly Notes due 2036, limited (except as otherwise provided in Article 2 of the Base Indenture) in aggregate principal amount to $100,000,000. The IQ Notes may be issued from time to time upon written order of the Company for the authentication and delivery of IQ Notes pursuant to Section 2.03 of the Base Indenture. Each of the Initial Guarantors (together with each other subsidiary of the Company that pursuant to the terms of the Indenture guarantees the Company’s obligations under the IQ Notes and the Indenture, the “Guarantors”) fully and unconditionally and jointly and severally guarantees to the Holders of the IQ Notes upon which the Guarantees are endorsed, upon authentication and delivery by the Trustee, the due and punctual payment of the principal of, and interest on, and any Redemption Price with respect to, the IQ Notes, when and as the same shall become due and payable, whether at the Maturity Date, upon acceleration or redemption or otherwise, in accordance with the terms of the IQ Notes and of the Indenture.

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SECTION 2.2. Maturity.

The date upon which the principal on the IQ Notes shall become due and payable at final maturity is October 1, 2036 (the “Maturity Date”) if not redeemed in full previously in accordance with Article III of this Fifth Supplemental Indenture.

SECTION 2.3. Form and Payment.

The IQ Notes shall be issued in fully registered certificated form without interest coupons, bearing identical terms (except as otherwise provided in Article 2 of the Base Indenture). Principal of, and interest on, and any Redemption Price with respect to, the IQ Notes will be payable, the transfer of such IQ Notes will be registrable and such IQ Notes will be exchangeable for IQ Notes bearing identical terms at the office or agency of the Company maintained for such purpose as described below.

The Company hereby designates the Borough of Manhattan, The City of New York as a place of payment (“Place of Payment”) for the IQ Notes, and the office or agency maintained by the Company in such Place of Payment for the purposes contemplated by this Section 2.3 shall initially be the Corporate Trust Office of the Trustee at 100 Wall Street, Suite 2000, New York, New York 10005, Attention: Richard Prokosch.

The IQ Notes shall be issuable in denominations of $1,000 and integral multiples of $1,000 in excess thereof.

The IQ Notes may be issued, in whole or in part, in permanent global form and, if issued in permanent global form, the Depository shall be The Depository Trust Company or such other depositary as any officer of the Company may from time to time designate.

The Registrar, the Paying Agent and the transfer agent for the IQ Notes shall initially be the Trustee.

The IQ Notes shall be in substantially the form set forth in Exhibit A hereto.

SECTION 2.4. Global Note.

(a) Unless and until it is exchanged for IQ Notes in registered certificated form, a global note in principal amount equal to the aggregate principal amount of the IQ Notes (the “Global Note”) may be transferred, in whole but not in part, only to the Depository or a nominee of the Depository, or to a successor Depository or to a nominee of such successor Depository.

(b) If at any time (i) the Depository notifies the Company that it is unwilling or unable to continue as a Depository for the Global Note and no successor Depository shall have been appointed within 90 days after such notification, (ii) the Depository ceases to be a clearing agency registered under the Securities Exchange Act of 1934 or any other applicable rule or regulation and no successor Depository shall have been appointed within 90 days after the Company becoming aware of the Depository’s ceasing to be so registered, (iii) the Company, in its sole discretion, determines

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that the Global Note shall be so exchangeable or (iv) there shall have occurred and be continuing an Event of Default, the Company will execute, and, subject to Article 2 of the Base Indenture, the Trustee, upon written notice from the Company, will authenticate and deliver IQ Notes, with the Guarantees endorsed thereon and executed by the Guarantors, in registered certificated form without coupons, in authorized denominations, and in an aggregate principal amount equal to the principal amount of the Global Note in exchange for such Global Note. Upon the exchange of the Global Note for such IQ Notes in registered certificated form without coupons, in authorized denominations, the Global Note shall be cancelled by the Trustee. Such IQ Notes in registered certificated form issued in exchange for the Global Note shall be registered in such names and in such authorized denominations as the Depository, pursuant to instructions from its direct or indirect participants or otherwise, shall instruct the Trustee. The Trustee shall deliver such IQ Notes to the Depository for delivery to the Persons in whose names such IQ Notes are so registered.

SECTION 2.5. Payment of Principal and Interest.

The IQ Notes shall bear interest at the per annum rate of 5.95%.

Interest shall be paid quarterly in arrears on each Interest Payment Date commencing on January 1, 2007. Payments of interest on the IQ Notes will include interest accrued from, and including, the immediately preceding Interest Payment Date to which interest has been paid or duly provided for (or from, and including, the Original Issue Date if no interest has been paid or duly provided for) to, but excluding, the applicable Interest Payment Date or date of earlier redemption, as the case may be. Interest payments for the IQ Notes shall be computed and paid on the basis of a 360-day year consisting of twelve 30-day months.

The interest so payable and punctually paid or duly provided for on any Interest Payment Date will be paid to the Holder(s) of the IQ Notes as of the Regular Record Date for such Interest Payment Date. Any such interest that is not so punctually paid or duly provided for on any Interest Payment Date will forthwith cease to be payable to the Holders of the IQ Notes as of the close of business on such Regular Record Date and may either be paid to the Person or Persons in whose name such IQ Notes are registered at the close of business on a Special Record Date for the payment of such defaulted interest to be fixed by the Trustee, notice whereof shall be given to Holders of the IQ Notes by the Trustee not less than fifteen (15) days prior to such Special Record Date, or be paid at any time in any other lawful manner, all as more fully provided in the Base Indenture.

Payment of the principal of, and any interest (other than interest due on an Interest Payment Date) on, the IQ Notes due on the Maturity Date or date of earlier redemption, as the case may be, shall be made in immediately available funds, upon presentation and surrender of the applicable IQ Notes at the office or agency maintained by the Company for that purpose in the Borough of Manhattan, The City of New York, currently the office of the Trustee located at 100 Wall Street, Suite 2000, New York, New York 10005, or at such other paying agency in the Borough of Manhattan, The City of New York, as the Company may determine. Payment of interest due on any Interest Payment Date will be made by wire transfer to the Holders entitled thereto of immediately

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available funds at such place and to such account at a banking institution in the United States as may be designated in wire transfer instructions received in writing by the Trustee at least sixteen (16) days prior to such Interest Payment Date or, if not so received, by check mailed to the address of the applicable Holders in the Security Register. Any such wire transfer instructions received by the Trustee shall remain in effect until revoked by such Holder.

Any payments on the IQ Notes will be made in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts.

In the event that any Interest Payment Date or the Maturity Date or date of earlier redemption falls on a day that is not a Business Day, the required payment of principal and/or interest payable on such date shall be made on the next succeeding Business Day with the same force and effect as if made on the date such payment was due, and no interest shall accrue with respect to such payment for the period from and after such Interest Payment Date or the Maturity Date or date of earlier redemption, as the case may be, to the date of such payment on the next succeeding Business Day.

SECTION 2.6. Events of Default

An “Event of Default” with respect to the IQ Notes means, in addition to those Events of Default set forth at Section 6.01 of the Base Indenture, the following:

(a) the occurrence of a declaration of acceleration with regard to the Securities of any other Series issued under the Base Indenture; or

(b) the occurrence of an event of default under the Insurance Agreement that is not waived or consented to by the Insurer.

ARTICLE III

REDEMPTION OF THE NOTES; DEFEASANCE

SECTION 3.1. Redemption at the Company’s Option.

The IQ Notes shall be subject to redemption at the option of the Company, in whole or in part, without premium or penalty, at any time in whole, and from time to time in part, on or after October 1, 2011. The IQ Notes shall also be subject to redemption at the option of the Company, (i) in whole or in part, in the event of a Transfer of a Guarantor under Section 10.02 of the Base Indenture, in an amount equal to the intercompany debt representing that portion of the IQ Notes previously loaned by the Company to such Guarantor that is repaid by such Guarantor to the Company pursuant to Section 10.02(1) of the Base Indenture minus any amount of such repayment that the Company loans to another Guarantor that is a signatory hereto, or (ii) in whole, if we have agreed to enter into a transaction that, upon the consummation thereof, would result in there no longer being at least one Guarantor which, as of the date hereof, is a public utility regulated by the Indiana Utility Regulatory Commission.

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The Company shall exercise such optional redemption upon not less than 30 nor more than 60 days’ notice by mail, at a redemption price (the “Redemption Price”) equal to 100% of the principal amount to be redeemed plus any unpaid interest accrued thereon to but excluding the applicable redemption date; provided, however, that interest payable on an IQ Note with respect to an Interest Payment Date that falls on or before such redemption date shall be made to the Holder thereof on the Regular Record Date related to such Interest Payment Date.

In the event of redemption of the IQ Notes in part only, a new IQ Note or IQ Notes for the unredeemed portion will be issued in the name or names of the Holders thereof upon the presentation and surrender thereof, as set forth in Section 3A.08 of the Base Indenture.

Notice of redemption shall be given as provided in Section 3A.05 of the Base Indenture.

Any redemption of less than all of the IQ Notes shall, with respect to the principal thereof, be divisible by $1,000.

SECTION 3.2. No Sinking Fund.

The IQ Notes are not subject to, or entitled to the benefit of, any sinking fund.

SECTION 3.3. Defeasance.

Article 8 of the Base Indenture describing Defeasance and Covenant Defeasance shall apply to the IQ Notes.

SECTION 3.4. Redemption upon Death of a Beneficial Owner.

Unless the IQ Notes have been declared due and payable prior to the Maturity Date by reason of an Event of Default, or have been previously redeemed or otherwise repaid in accordance with their terms, the Representative (as hereinafter defined) of a deceased Beneficial Owner (as hereinafter defined) of the IQ Notes has the right to request redemption prior to the Maturity Date of all or part of his or her IQ Notes, and the Company will redeem the same, subject to the limitations that the Company will not be obligated to redeem, during the period from the Original Issue Date through and including October 1, 2007 (the “Initial Period”), and, during any twelve-month period that ends on and includes each October 1 thereafter (each such twelve-month period being hereinafter referred to as a “Subsequent Period”), on behalf of a deceased Beneficial Owner, (i) IQ Notes with an aggregate principal amount in excess of $25,000 or (ii) IQ Notes exceeding $2,000,000 in aggregate principal amount from all deceased Beneficial Owners.

The Company may, at its option, redeem any deceased Beneficial Owner’s IQ Notes in the Initial Period or any Subsequent Period in excess of the $25,000 limitation. Any such redemption by the Company, to the extent that it exceeds the $25,000 limitation for any deceased Beneficial Owner, shall not be included in the computation of the $2,000,000 aggregate limitation for the IQ Notes for such Initial Period or such Subsequent Period, as the case may be, or for any succeeding Subsequent

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Period. The Company may, at its option, redeem deceased Beneficial Owners’ IQ Notes in the Initial Period or in any Subsequent Period in an aggregate principal amount exceeding the $2,000,000 aggregate limitation. Any such redemption by the Company, to the extent it exceeds the $2,000,000 aggregate limitation, shall not reduce the $2,000,000 aggregate limitation for any succeeding Subsequent Period. On any determination by the Company to redeem IQ Notes in excess of the $25,000 limitation or the $2,000,000 aggregate limitation, such IQ Notes shall be redeemed in the order of the receipt of Redemption Requests (as hereinafter defined) by the Trustee.

A request for redemption of an IQ Note may be initiated by the personal representative or other person authorized to represent the estate of the deceased Beneficial Owner or from a surviving joint tenant(s) or tenant(s) by the entirety or the trustee of a trust (each, a “Representative”). A Representative may initiate a request for redemption at any time and in any principal amount, provided that the principal amount is in integral multiples of $1,000. The Representative shall deliver its request to the Participant (as hereinafter defined) through whom the deceased Beneficial Owner owned the Note to be redeemed, in form satisfactory to the Participant, together with evidence of the death of such Beneficial Owner, evidence of the authority of the Representative satisfactory to the Participant, any waivers, notices or certificates as may be required under applicable state or federal law and any other evidence of the right to such redemption as the Participant requires. The request must specify the principal amount of the IQ Notes to be redeemed. Subject to the rules and arrangements applicable to the Depository, the Participant will then deliver to the Depository a request for redemption substantially in the form attached to the IQ Notes as Annex A (a “Redemption Request”). The Depository will, upon receipt of a Redemption Request, forward the same to the Trustee. The Trustee is required to maintain records with respect to Redemption Requests received by it, including the date of receipt, the name of the Participant filing the Redemption Request and the status of each Redemption Request with respect to the $25,000 limitation and the $2,000,000 aggregate limitation. The Trustee will immediately file with the Company each Redemption Request it receives, together with the information regarding the eligibility of the Redemption Request with respect to the $25,000 limitation and the $2,000,000 aggregate limitation. The Company, the Depository and the Trustee may conclusively assume, without independent investigation, that the statements contained in each Redemption Request are true and correct and shall have no responsibility (a) for reviewing any documents submitted to the Participant by the Representative or (b) for determining whether the applicable decedent is in fact the Beneficial Owner of the interest in the IQ Notes to be redeemed or is in fact deceased and whether the Representative is duly authorized to request redemption on behalf of the applicable Beneficial Owner.

Subject to the $25,000 limitation and the $2,000,000 aggregate limitation, the Company will, after the death of any Beneficial Owner, redeem such Beneficial Owner’s IQ Note on the next Interest Payment Date occurring not less than 30 days following receipt by the Company of a Redemption Request from the Trustee. If Redemption Requests exceed the $2,000,000 aggregate limitation during the Initial Period or any Subsequent Period, then such excess Redemption Requests will be applied, in the order received by the Trustee to successive Subsequent Periods, regardless of the number of Subsequent Periods required to redeem such interests. The Company may, at any time,

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notify the Trustee that it will redeem, on the next Interest Payment Date not less than 30 days thereafter, all or any lesser amount of IQ Notes for which Redemption Requests have been received but which are not then eligible for redemption by reason of the $25,000 limitation or the $2,000,000 aggregate limitation. Any IQ Notes so redeemed shall be redeemed in the order of receipt of Redemption Requests by the Trustee.

The price to be paid by the Company for the IQ Notes to be redeemed pursuant to a Redemption Request is 100% of the principal amount thereof plus any unpaid interest accrued thereon to but excluding the applicable redemption date; provided, however, that interest payable on an IQ Note with respect to an Interest Payment Date that falls on or before such redemption date shall be made to the Holder thereof on the Regular Record Date related to such Interest Payment Date. Subject to arrangements with the Depository, payment for IQ Notes to be redeemed shall be made to the Depository upon presentation of the IQ Notes to the Trustee for redemption in the aggregate principal amount specified in the Redemption Requests submitted to the Trustee by the Depository which are to be fulfilled in connection with such redemption. The principal amount of any IQ Notes acquired or redeemed by the Company other than by redemption at the option of any Representative of a deceased Beneficial Owner pursuant to this Section 3.4 shall not be included in the computation of either the $25,000 limitation or the $2,000,000 aggregate limitation for the Initial Period or any Subsequent Period.

For purposes of this section, a “Beneficial Owner” means the person who has the right to sell, transfer or otherwise dispose of an IQ Note and the right to receive the proceeds therefrom, as well as the interest and principal payable to the Holder thereof. In general, a determination of beneficial ownership in the IQ Notes will be subject to the rules, regulations and procedures governing the Depository and institutions (“Participants”) that have accounts with the Depository or a nominee thereof.

For purposes of this section, an IQ Note held in tenancy by the entirety, by joint tenancy or by tenants in common will be deemed to be held by a single Beneficial Owner, and the death of a tenant by the entirety, joint tenant or tenant in common will be deemed the death of a Beneficial Owner. The death of a person who, during his or her lifetime, was entitled to substantially all of the rights of a Beneficial Owner of an interest in the IQ Notes will be deemed the death of the Beneficial Owner, regardless of the recordation of such ownership on the records of the Participant, if such rights can be established to the satisfaction of the Participant and the Company.

In the case of a Redemption Request that is presented on behalf of a deceased Beneficial Owner and that has not been fulfilled at the time the Company gives notice of its election to redeem the IQ Notes pursuant to Section 3.1 hereof, the IQ Notes that are the subject of such pending Redemption Request shall be redeemed pursuant to Section 3.1 hereof prior to any other IQ Notes.

Any Redemption Request may be withdrawn by the person(s) presenting such request upon delivery of a written request for such withdrawal given by the Participant on behalf of such person(s) to the Depository and by the Depository to the Trustee not less than 60 days prior to the Interest Payment Date on which such IQ Notes are first eligible for redemption.

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The Company may, at its option, purchase any IQ Notes for which Redemption Requests have been received in lieu of redeeming such IQ Notes. Any IQ Notes so purchased by the Company shall be presented to the Trustee for redemption and cancellation.

During such time or times as the IQ Notes are not represented by a Global Security and are issued in certificated form, all references to Participants and the Depository, including the Depository’s governing rules, regulations and procedures, shall be deemed deleted, all determinations which under this Section the Participants are required to make shall be made by the Company (including, without limitation, determining whether the applicable decedent is in fact the Beneficial Owner of the interest in the IQ Notes to be redeemed or is in fact deceased and whether the Representative is duly authorized to request redemption on behalf of the applicable Beneficial Owner), and all Redemption Requests, to be effective, shall be delivered by the Representative to the Trustee, with a copy to the Company, and shall be in the form of a Redemption Request (with appropriate changes mutually agreed to by the Trustee and the Company to reflect the fact that such Redemption Request is being executed by a Representative (including provision for signature guarantees)) and, in addition to all documents that are otherwise required to accompany a Redemption Request, shall be accompanied by the IQ Note that is the subject of such request and, if applicable, a properly executed assignment or endorsement. If the Holder of the IQ Note is held by a nominee of the deceased Beneficial Owner, a certificate or letter from such nominee attesting to the deceased’s ownership of a beneficial interest in the IQ Note must also be delivered.

ARTICLE IV

SPECIAL INSURANCE PROVISIONS

SECTION 4.1. Supplemental Indentures.

For so long as the Policy is in effect (and the Insurer is not in default thereunder) or any obligation by the Company or a Guarantor to reimburse the Insurer for any payment made by the Insurer under the Policy remains unpaid, the consent of the Insurer shall be required with respect to any amendment or supplement to the Indenture or Guarantees, except that the Insurer’s consent shall not be required for any supplemental indenture entered