Exhibit 10.4

EXECUTION COPY

PLEDGE AND SECURITY AGREEMENT

PLEDGE AND SECURITY AGREEMENT dated as of August 30, 2004 made by Affiliated Managers Group, Inc., a Delaware corporation (the “ Pledgor ”), to The Bank of New York, as agent (together with any successor agent, the “ Agent ”) for the Bridge Lenders (as defined below) (the Agent and the Bridge Lenders, collectively, the “ Secured Parties ”).

RECITALS

(1)           The Pledgor has consummated its offer to purchase for cash any and all of its outstanding 6% Senior Notes due 2006, originally issued by the Pledgor as a constituent of the Income PRIDES (the “Senior Notes” ), by purchasing all of the Senior Notes tendered pursuant to the terms thereof (such offer to purchase, the “ Tender Offer ”).  Pursuant to the terms of the Tender Offer, $50,744,135.04 of the cash payable to tendering holders of such Senior Notes has been used to purchase principal strips of U.S. Treasury securities with the CUSIP No. 912803AB9 (all such purchased U.S. Treasury securities, the “ Treasury Strips ”).  Upon consummation of the Tender Offer, the Pledgor issued Growth PRIDES, CUSIP No. 008252603 (the “ Growth PRIDES ”), to the tendering holders of such Senior Notes that were not previously separated from the related Income PRIDES, which securities consist of (a) Purchase Contracts (as such term is defined the Purchase Contract Agreement, dated as of December 21, 2001 (such agreement, as amended, supplemented or otherwise modified in accordance with its terms from time to time, the “ Purchase Contract Agreement ”)), between the Pledgor and The Bank of New York (as successor purchase contract agent pursuant to the Instrument of Resignation, Appointment and Acceptance dated January 15, 2003 among the Company, Wachovia Bank, National Association, and The Bank of New York (the “ Instrument of Registration ”)), as purchase contract agent thereunder (The Bank of New York in such capacity, the “ Purchase Contract Agent ” and such Purchase Contracts, the “ Growth PRIDES Purchase Contracts ”), and (b) that number of Treasury Strips comprising the amount required to be delivered to release the Senior Notes from the pledge arrangement under the Income PRIDES (such Treasury Strips, the “ Conveyed Securities ”).  Pursuant to the terms of the Purchase Contract Agreement, the Conveyed Securities are subject to the security interest granted to the Pledgor under the Pledge Agreement, dated as of December 21, 2001 (as amended, supplemented or otherwise modified in accordance with its terms from time to time, the “ Holders Pledge Agreement ”), between the Pledgor and The Bank of New York (as successor collateral agent, custodial agent, securities intermediary and Purchase Contract Agent pursuant to the Instrument of Resignation), as collateral agent, custodial agent, securities intermediary and purchase contract agent thereunder, which security interest in the Conveyed Securities secures, inter alia , the payment and performance of the Growth PRIDES Purchase Contracts.

(2)           Pursuant to the Pledge Agreement, the Pledgor has security entitlements (the “ Pledged Security Entitlements ”) with respect to financial assets (the “ Pledged Financial Assets ”) (a) that consist of all cash and other property credited from time to time to Pledgor’s account, account no. 293629, maintained in the name “The Bank of New York, as agent, for the secured parties under the Pledge Agreement dated as of August 30, 2004 made by Affiliated

Managers Group, Inc.” (such account and any successor securities accounts thereto, the “ Pledgor’s Securities Account ”), with BNY at its office at 101 Barclay Street, Fl. 8W, New York, New York, and (b) that consist of the Conveyed Securities and all cash and other property paid or otherwise delivered in respect thereof and credited from time to time to the account maintained on behalf of the holders of Growth PRIDES, account no. 293627 in the name “The Bank of New York, as Purchase Contract Agent on behalf of the holders of certain securities of Affiliated Managers Group, Inc., Collateral Account subject to the security interest of The Bank of New York, as Collateral Agent, for the benefit of Affiliated Managers Group, Inc., as pledgee” (such account and any successor securities accounts thereto, the “ PRIDES Securities Account ”, and together with the Pledgor’s Securities Account, the “ Securities Accounts ” and each a “ Securities Account ”), with BNY at its office at 101 Barclay Street, Fl. 8W, New York, New York.

(3)           The Pledgor is entering into a Credit Agreement, dated as of the date hereof (said agreement, as it may hereafter be amended, amended and restated, supplemented or otherwise modified from time to time, the “ Bridge Loan Agreement ”), among the Pledgor, as borrower, the Agent, as administrative agent, and the other lenders from time to time parties thereto (collectively, the “ Bridge Lenders ”).  Pursuant to the Bridge Loan Agreement, the Bridge Lenders have agreed to make loans and otherwise extend credit to the Pledgor, the proceeds of which loans may be used by the Pledgor to finance or refinance the purchase by the Pledgor of the Senior Notes pursuant to the Tender Offer.  It is a condition precedent to the Bridge Lenders making any loans or otherwise extending any credit to the Pledgor that the Pledgor execute and deliver to the Agent this Agreement for the benefit of the Secured Parties.

(5)           Unless otherwise defined in this Agreement, terms defined in Article 8 or 9 of the Uniform Commercial Code in effect in the State of New York (the “ UCC ”) and/or the Federal Book Entry Regulations (as defined below) are used in this Agreement as such terms are defined in such Article 8 or 9 and/or the Federal Book Entry Regulations.  The term “Federal Book Entry Regulations” means (a) the federal regulations contained in Subpart B (“Treasury/Reserve Automated Debt Entry System (TRADES)”) governing book-entry securities consisting of U.S. Treasury bonds, notes, bills and other securities (as defined therein) and Subpart D (“Additional Provisions”) of 31 C.F.R Part 357, 31 C.F.R. §357.2, §357.10 through § 357.14 and §357.44 and (b) to the extent substantially identical to the federal regulations referred to in clause (a) above (as in effect from time to time), the federal regulations governing other book-entry securities.

NOW, THEREFORE, in consideration of the premises contained herein and for other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereto agree as follows:

Section 1.  Grant of Security .  The Pledgor hereby assigns and pledges to the Agent for the ratable benefit of the Secured Parties, and hereby grants to the Agent for the ratable benefit of the Secured Parties a security interest in, the Pledgor’s right, title and interest in and to the following, in each case whether now owned or hereafter acquired by the Pledgor and whether now or hereafter existing or arising (collectively, the “ Collateral ”):

(a)         the Securities Accounts, the Pledged Security Entitlements with respect to the Pledged Financial Assets from time to time credited to any Securities Account, and all

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interest, cash, instruments and other property from time to time received, receivable or otherwise distributed in respect of or in exchange for any or all of such Pledged Security Entitlements or such Pledged Financial Assets;

(b)        all accounts, payment intangibles and other general intangibles of the Pledgor arising in respect of the Growth PRIDES Purchase Contracts;

(c)         the following rights and claims under and with respect to each of the Purchase Contract Agreement and the Holders Pledge Agreement (collectively, the “ Assigned Agreements ”) and the Growth PRIDES Purchase Contracts: (i) all rights of the Pledgor to receive moneys due and to become due under or pursuant to the Assigned Agreements and the Growth PRIDES Purchase Contracts, (ii) all rights of the Pledgor to receive proceeds of any indemnity, warranty or guaranty, (iii) all claims of the Pledgor for damages arising out of or for breach of or default under the Assigned Agreements and the Growth PRIDES Purchase Contracts, and (iv) the right of the Pledgor to compel performance and otherwise exercise all remedies under the Assigned Agreements and the Growth PRIDES Purchase Contracts, in the case of each of subclauses (i) through (iv) with respect to the Assigned Agreements, solely with respect to the Collateral described in the preceding clauses (a) and (b);

(d)        all proceeds of any and all of the Collateral (including, without limitation, proceeds that constitute property of the types described in clauses (a) through (c) of this Section 1 and this clause (d)) and, to the extent not otherwise included, all (i) payments with respect to any of the foregoing Collateral and (ii) cash received in respect of any of the foregoing Collateral; provided that the inclusion of such proceeds in the Collateral shall not be deemed to be a consent by the Secured Parties to any sale or other disposition of any Collateral not otherwise specifically permitted or contemplated by the terms hereof or of the other Loan Documents.

Notwithstanding anything to the contrary herein, the security interest granted under this Section 1 is subject to the terms of the Assigned Agreements and the Growth PRIDES Purchase Contracts, including, without limitation, Article V of the Purchase Contract Agreement.

Section 2.  Security for Obligations .  This Agreement secures the payment and performance of all Obligations (as defined below) now or hereafter existing, whether direct or indirect, absolute or contingent, and whether for principal, reimbursement obligations, interest, fees, premiums, penalties, indemnifications, contract causes of action, costs, expenses or otherwise.  For the purposes of this Agreement, “ Obligations ” shall mean, (a) the full and prompt payment when due (whether at the stated maturity, by acceleration or otherwise) of all obligations of the Pledgor now existing or hereafter incurred under, arising out of or in connection with the Bridge Loan Agreement, the promissory note(s) issued by the Pledgor thereunder, and each other agreement, instrument and document delivered thereunder or in connection therewith (collectively, the “ Loan Documents ”) and the due performance and compliance by the Pledgor with the terms of each such Loan Document; (b) any and all sums advanced by the Agent in order to preserve the Collateral or to preserve its security interest in the Collateral; (c) in the event of any proceeding for the collection or enforcement of any obligations or liabilities, after an Event of Default (as defined below) shall have occurred and be continuing, the reasonable expenses of holding, preparing for sale, selling or otherwise disposing of or

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realizing on the Collateral, or of any exercise by the Agent of its rights hereunder, together with reasonable attorneys’ fees and court costs; and (d) all amounts paid by any indemnitee as to which such indemnitee has the right to reimbursement under this Agreement.

Section 3.  Delivery and Control of Collateral .  With respect to any Collateral in which the Pledgor has any right, title or interest and that constitutes a security entitlement (including, without limitation, the Pledged Securities Entitlements), the Pledgor will cause the securities intermediary with respect to such security entitlement either (i) to identify in its records the Agent as the entitlement holder of such security entitlement against such securities intermediary or (ii) to agree in writing with the Pledgor and the Agent that such securities intermediary will comply with entitlement orders originated by the Agent without further consent of the Pledgor, such agreement to be in substantially the form of Exhibit A hereto or otherwise in form and substance satisfactory to the Agent (such agreement being a “ Control Agreement ”).

Section 4.  Representations and Warranties .  The Pledgor represents and warrants as follows:

(a)         The Pledgor is the legal and beneficial owner of the Collateral (other than the interests of the holders of Growth PRIDES in the Treasury Strips credited to the PRIDES Securities Account) free and clear of any Lien, claim, option or right of others, except for the security interest created under this Agreement and the Liens created thereon pursuant to the Holders Pledge Agreement.  No effective financing statement or other instrument similar in effect covering all or any part of such Collateral or listing the Pledgor as debtor is on file in any recording office, except such as may have been filed in favor of the Agent relating to the Loan Documents or as otherwise relating to the Liens created thereon pursuant to the Holders Pledge Agreement.

(b)        The Assigned Agreements, true and complete copies of which have been furnished to the Agent, and the Growth PRIDES Purchase Contracts have been duly authorized, executed and delivered by all parties thereto, have not been amended, amended and restated, supplemented or otherwise modified except as evidenced by such copies so furnished, are in full force and effect and are binding upon and enforceable against all parties thereto in accordance with their terms.  There exists no default under any Assigned Agreement or Growth PRIDES Purchase Contract by any party thereto.

(c)         All of the investment property constituting Pledged Financial Assets in which the Pledgor has or will have any interest are and will be maintained in the Securities Accounts.

(d)        All filings and other actions necessary or desirable to perfect and protect the security interest in the Collateral created under this Agreement have been duly made or taken and are in full force and effect, and this Agreement creates in favor of the Agent for th