<PAGE>

                                                                 EXHIBIT 10.36.5
                                 AMENDMENT NO. 4

                            dated as of April 1, 2003


                                      among


                     AMERICREDIT MTN RECEIVABLES TRUST III,

                                   as Debtor,


                      AMERICREDIT FINANCIAL SERVICES, INC.,

                          Individually and as Servicer,


                           MBIA INSURANCE CORPORATION,

                                   as Insurer


                                       and


                         MERIDIAN FUNDING COMPANY, LLC,

                                  as Purchaser


                              to SECURITY AGREEMENT

                          dated as of February 25, 2002

<PAGE>

        THIS AMENDMENT NO. 4 TO THE SECURITY AGREEMENT, dated as of April 1,
2003 (this "Amendment") is among AMERICREDIT MTN RECEIVABLES TRUST III (the
"Debtor"), AMERICREDIT FINANCIAL SERVICES, INC., individually and in its
capacity as Servicer ("AFS"), MBIA INSURANCE CORPORATION, as Insurer ("MBIA"),
and MERIDIAN FUNDING COMPANY, LLC, as Purchaser ("Meridian"), to the Security
Agreement dated as of February 25, 2002 (the "Security Agreement").

        WHEREAS, the Debtor, AFS, AFS AmeriCredit MTN Corp. III and JPMorgan
Chase Bank (successor to Chase Manhattan Bank) entered into a Security Agreement
dated as of February 25, 2002 relating to the $500,000,000 AmeriCredit MTN
Receivables Trust III Note;

        WHEREAS, Section 9.2(b) of the Security Agreement permits amendment of
the Security Agreement by the Debtor, AFS, MBIA and Meridian (the "Parties")
upon the terms and conditions specified therein;

        WHEREAS, the Security Agreement has previously been amended by Amendment
No. 1, dated as of December 1, 2002, Amendment No. 2, dated as of February 1,
2003 and Amendment No. 3, dated as of February 28, 2003 among the Parties;

        WHEREAS, the Insurer is the Controlling Party under the Transaction
Documents;

        WHEREAS, the Parties wish to amend the Security Agreement as provided
herein.

        NOW, THEREFORE, for good and valuable consideration, the receipt and
adequacy of which are hereby acknowledged, the Parties agree that the Security
Agreement is hereby amended effective as of the date hereof as follows:

        Section 1. Definitions Each term used herein but not defined herein
shall have the meaning assigned to such term in the Security Agreement.

        Section 2. Amendment to Section 1.1 (Certain Defined Terms)

        (a) The definition of "Credit Score Based Reserve Percentage" in Section
        1.1 is amended and restated, in its entirety, as follows:

        "Credit Score Based Reserve Percentage" means, with respect to the
        Amortization Period, the applicable percentage set forth in the number
        clauses (i) through (v) below:

                        (i) 13% if the Weighted Average AmeriCredit Score of all
                Eligible Receivables as of the commencement of the Amortization
                Period, after taking into account all Facility Activity on such
                date is greater than or equal to 235.00; or

                        (ii) 14% if the Weighted Average AmeriCredit Score of
                all Eligible Receivables as of the commencement of the
                Amortization Period, after taking into account all Facility
                Activity on such date is greater than or equal to 234 but less
                than 235; or

<PAGE>

                        (iii) 15% if the Weighted Average AmeriCredit Score of
                all Eligible Receivables as of the commencement of the
                Amortization Period, after taking into account all Facility
                Activity on such date is greater than or equal to 232 but less
                than 234; or

                        (iv) 16% if the Weighted Average AmeriCredit Score of
                all Eligible Receivables as of the commencement of the
                Amortization Period, after taking into account all Facility
                Activity on such date is greater than or equal to 230 but less
                than 232; or

                        (v) 17% if the Weighted Average AmeriCredit Score of all
                Eligible Receivables as of the commencement of the Amortization
                Period, after taking into account all Facility Activity on such
                date is greater than or equal to is less than 230.

        (b) The definition of "Initial Reserve Percentage" in Section 1.1 is
        amended and restated, in its entirety, as follows:

        "Initial Reserve Percentage" means, with respect to any Delivery Date:

                        (i) 6%, if both

                        (a) the Weighted Average AmeriCredit Score of all
                Eligible Receivables (including the Receivables to be delivered
                on such Receivables Delivery Date) is greater than or equal to
                235.00; and

                        (b) the most-recently calculated Portfolio Net Loss
                Ratio is less than 7.00%

                        (ii) 7%, if either

                        (a) the Weighted Average AmeriCredit Score of all
                Eligible Receivables (including the Receivables to be delivered
                on such Receivables Delivery Date) is greater than or equal to
                234.00 but less than 235.00; or

                        (b) the most-recently calculated Portfolio Net Loss
                Ratio is greater than or equal to 7.00% but less than 7.50%.

                        (iii) 8%, if either


                        (a) the Weighted Average AmeriCredit Score of all
                Eligible Receivables (including the Receivables to be delivered
                on such Receivables Delivery Date) is less than 234.00; or

                        (b) the most-recently calculated Portfolio Net Loss
                Ratio is greater than or equal to 7.50% but less than 8.00%.

<PAGE>

                        (iv) 9%, if

                        (a) the most-recently calculated Portfolio Net Loss
                Ratio is greater than or equal to 8.00%.

        (c) The definition of "Premium Side Letter" in Section 1.1 is amended
        and restated, in its entirety, as follows:

                "Premium Side Letter" means the letter of agreement dated June
        11, 2001 between AmeriCredit and the Note Insurer, as amended, modified
        or otherwise supplemented from time to time.

        (d) The definition of "Revolving Period Reserve Percentage" in Section
        1.1 is amended and restated, in its entirety, as follows:

                "Revolving Period Reserve Percentage" means, with respect to any
        Borrowing Base Determination Date during the Revolving Period, the sum
        of (x) if, and only if, the Weighted Average Age of the Eligible
        Receivables then exceeds 180.00 days, 3%, plus (y) the applicable
        percentage set forth in the numbered clauses (i) through (iv) below:

                (i) 8%, if both

                        (a) the Weighted Average AmeriCredit Score of all
                        Eligible Receivables as of such date of determination,
                        after taking into account all Facility Activity on such
                        date is greater than or equal to 235.00; and

                        (b) the most-recently calculated Portfolio Net Loss
                        Ratio is less than 7.00%.

                (ii) 9%, if either

                        (a) the Weighted Average AmeriCredit Score of all
                        Eligible Receivables as of such date of determination,
                        after taking into account all Facility Activity on such
                        date is greater than or equal to 234.00 but less than
                        235.00; or

                        (b) the most-recently calculated Portfolio Net Loss
                        Ratio is greater than or equal to 7.00% but less than
                        7.50%.

                (iii) 10%, if either

                        (a) the Weighted Average AmeriCredit Score of all
                        Eligible Receivables as of such date of determination,
                        after taking into account all Facility Activity on such
                        date is greater than or equal to 232.00 but less than
                        234.00; or

<PAGE>

                        (b) the most-recently calculated Portfolio Net Loss
                        Ratio is greater than or equal to 7.50% but less than
                        8.00%.

                (iv) 11%, if either

                        (a) the Weighted Average AmeriCredit Score of all
                        Eligible Receivables as of such date of determination,
                        after taking into account all Facility Activity on such
                        date is less than 232.00; or

                        (b) the most-recently calculated Portfolio Net Loss
                        Ratio is greater than or equal to 8.00%.

        Section 3. Amendment to Section 6.1 (Termination and Amortization
Events)

        Section 6.1(nn) is amended and restated, in its entirety, as follows:

                (nn) The portion of AmeriCredit's total committed and in good
        standing warehouse facilities that is insured by the Note Insurer
        exceeds 40%.

        Section 4. Separate Counterpart. This Amendment may be executed by the
parties hereto in separate counterparts, each of which when so executed and
delivered shall be an original, but all such counterparts shall together
constitute but one and the same instrument.

        Section 5. Governing Law. THIS AMENDMENT SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD
TO ITS CONFLICTS OF LAW PROVISIONS.

        Section 6. Binding Effect; Ratification.

                (a) This Amendment shall become effective as of the date first
        set forth above, when counterparts hereof shall have been executed and
        delivered by the parties hereto, and thereafter shall be binding on the
        parties hereto and their respective successors and assigns.

                (b) On and after the execution and delivery hereof, (i) this
        Amendment shall be a part of the Security Agreement, and (ii) each
        reference in any Transaction Document (as defined in the Insurance
        Agreement) to the Security Agreement shall mean and be a reference to
        the Security Agreement as amended hereby.

                (c) Except as expressly amended hereby, all provisions of the
        Security Agreement shall remain in full force and effect and are hereby
        ratified and confirmed by the parties hereto.

                            [Signature Pages Follow]

<PAGE>

        IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the date set forth on the first page hereof.


                                        AMERICREDIT MTN RECEIVABLES TRUST III

                                        By: DEUTSCHE BANK TRUST COMPANY
                                         DELAWARE, not in its individual
                                         capacity but solely as Owner Trustee on
                                         behalf of the Issuer

                                        By:
                                            ----------------------------
                                            Name:
                                            Title:


                                        AMERICREDIT FINANCIAL SERVICES, INC.,
                                         Individually and as Servicer

                                        By:
                                            ----------------------------
                                            Name:
                                            Title:


                                        MBIA INSURANCE CORPORATION,
                                         as Insurer

                                        By
                                            ----------------------------
                                            Name:
                                            Title:


                                        MERIDIAN FUNDING COMPANY, LLC,
                                         as Purchaser

                                        By
                                            ----------------------------
                                            Name:
                                            Title:

Signature Page for Amendment No. 4
to the Security Agreement (MTN III)