<PAGE>   1
                                                                  EXHIBIT 4.8(d)

                                 TRUST AGREEMENT
                                       OF
                            INDYMAC CAPITAL TRUST IV

        THIS TRUST AGREEMENT OF INDYMAC CAPITAL TRUST IV is dated as of August
16, 2001 (this "Trust Agreement"), by and among IndyMac Bancorp, Inc., a
Delaware corporation, as depositor (the "Depositor"), Wilmington Trust Company,
a Delaware banking corporation, as trustee (the "Property Trustee"), Wilmington
Trust Company, a Delaware banking corporation, as trustee (the "Delaware
Trustee"), and Roger H. Molvar, as trustee (the "Administrative Trustee" and
collectively with the Property Trustee and the Delaware Trustee, the
"Trustees"). The Depositor and the Trustees hereby agree as follows:

        1. The trust created hereby shall be known as "IndyMac Capital Trust IV"
(the "Trust"), in which name the Trustees or the Depositor, to the extent
provided herein, may conduct the business of the Trust, make and execute
contracts, and sue and be sued.

        2. The Depositor hereby assigns, transfers, conveys and sets over to the
Trust the sum of $10. The Trustees hereby acknowledge receipt of such amount
from the Depositor, which amount shall constitute the initial trust estate. The
Trustees hereby declare that they will hold the trust estate for the Depositor.
It is the intention of the parties hereto that the Trust created hereby
constitute a business trust under Chapter 38 of Title 12 of the Delaware Code,
12 Del. C. Section 3801, et seq. (the "Business Trust Act"), and that this
document constitute the governing instrument of the Trust. The Trustees are
hereby authorized and directed to execute and file a certificate of trust with
the Delaware Secretary of State in such form as the Trustees may approve.

        3. The Depositor and the Trustees will enter into an amended and
restated Trust Agreement satisfactory to each such party to provide for the
contemplated operation of the Trust created hereby and the issuance of the
preferred or capital securities and common securities referred to therein. Prior
to the execution and delivery of such amended and restated Trust Agreement, the
Trustees shall not have any duty or obligation hereunder or with respect of the
trust estate, except as otherwise contemplated by this Trust Agreement, required
by applicable law or as may be necessary to obtain prior to such execution and
delivery any licenses, consents or approvals required by applicable law or
otherwise. Notwithstanding the foregoing, the Trustees may take all actions
deemed proper as are necessary to effect the transactions contemplated herein.

        4. The Depositor, as depositor of the Trust, or the Administrative
Trustee, acting singly or jointly, is hereby authorized (i) to prepare and file
with the Securities and Exchange Commission (the "Commission") and to execute,
in the case of the 1933 Act Registration Statement and 1934 Act Registration
Statement (as herein defined), on behalf of the Trust, (a) a Registration
Statement (the "1933 Act Registration Statement"), including all pre-effective
and post-effective amendments thereto, relating to the registration under the
Securities Act of 1933, as amended (the "1933 Act"), of the preferred or capital
securities of the Trust, (b) any preliminary prospectus or prospectus or
supplement thereto relating to the preferred or capital securities of the Trust
required to be filed pursuant to the 1933 Act, and (c) a Registration Statement
on Form 8-A or other appropriate form (the "1934 Act Registration Statement"),
including all pre-


<PAGE>   2

effective and post-effective amendments thereto, relating to the registration of
the preferred or capital securities of the Trust under the Securities Exchange
Act of 1934, as amended; (ii) if and at such time as determined by the
Depositor, to file with the New York Stock Exchange or other exchange, or the
National Association of Securities Dealers ("NASD"), and execute on behalf of
the Trust a listing application and all other applications, statements,
certificates, agreements and other instruments as shall be necessary or
desirable to cause the preferred or capital securities of the Trust to be listed
on the New York Stock Exchange or such other exchange, or the NASD's Nasdaq
National Market; (iii) to file and execute on behalf of the Trust, such
applications, reports, surety bonds, irrevocable consents, appointments of
attorney for service of process and other papers and documents that shall be
necessary or desirable to register the preferred or capital securities of the
Trust under the securities or "blue sky" laws of such jurisdictions as the
Depositor, on behalf of the Trust, may deem necessary or desirable; (iv) to
execute and deliver letters or documents to, or instruments for filing with, a
depository relating to the preferred or capital securities of the Trust; and (v)
to execute, deliver and perform on behalf of the Trust an underwriting agreement
with one or more underwriters relating to the offering of the preferred or
capital securities of the Trust.

        In the event that any filing referred to in this Section 4 is required
by the rules and regulations of the Commission, the New York Stock Exchange or
other exchange, NASD, or state securities or "blue sky" laws to be executed on
behalf of the Trust by the Trustees, the Trustees, in their capacity as trustees
of the Trust, are hereby authorized to join in any such filing and to execute on
behalf of the Trust any and all of the foregoing, it being understood that the
Trustees, in their capacity as trustees of the Trust, shall not be required to
join in any such filing or execute on behalf of the Trust any such document
unless required by the rules and regulations of the Commission, the New York
Stock Exchange or other exchange, NASD, or state securities or "blue sky" laws.

        5. This Trust Agreement may be executed in one or more counterparts.

        6. The number of trustees of the Trust initially shall be three and
thereafter the number of trustees of the Trust shall be such number as shall be
fixed from time to time by a written instrument signed by the Depositor which
may increase or decrease the number of trustees of the Trust; provided, however,
that to the extent required by the Business Trust Act, one trustee of the Trust
shall either be a natural person who is a resident of the State of Delaware or,
if not a natural person, an entity which has its principal place of business in
the State of Delaware and otherwise meets the requirements of applicable law.
Subject to the foregoing, the Depositor is entitled to appoint or remove without
cause any trustee of the Trust at any time. Any trustee of the Trust may resign
upon thirty days' prior notice to the Depositor.

        7. The Depositor hereby agrees to (i) reimburse the Trustees for all
reasonable expenses (including reasonable fees and expenses of counsel and other
experts) and (ii) indemnify, defend and hold harmless the Trustees and any of
the officers, directors, employees and agents of the Trustees (the "Indemnified
Persons") from and against and all losses, damages, liabilities, claims,
actions, suits, costs, expenses, disbursements (including the reasonable fees
and expenses of counsel), taxes and penalties of any kind and nature whatsoever
(collectively, "Expenses"), to the extent that such Expenses arise out of or are
imposed upon or asserted at any time against such Indemnified Persons with
respect to the performance of this Trust Agreement, the creation, operation or
termination of the


                                       2
<PAGE>   3

Trust or the transactions contemplated hereby; provided, however, that the
Depositor shall not be required to indemnify any Indemnified Person for any
Expenses which are a result of the willful misconduct, bad faith or gross
negligence of such Indemnified Person.

        8. The Trust may be dissolved and terminated before the issuance of the
preferred or capital securities of the Trust at the election of the Depositor.

        9. This Trust Agreement shall be governed by, and construed in
accordance with, the laws of the State of Delaware (without regard to conflict
of laws principles).

        IN WITNESS WHEREOF, the parties hereto have caused this Trust Agreement
to be duly executed as of the day and year first above written.

                                           INDYMAC BANCORP, INC., as Depositor


                                           By:    /s/ Roger H. Molvar
                                              ----------------------------------
                                           Name:  Roger H.Molvar
                                           Title: Executive Vice President and
                                                  Chief Administrative Officer


                                           WILMINGTON TRUST COMPANY, as Property
                                           Trustee and Delaware Trustee


                                           By: /s/ Anita Dallago
                                              ----------------------------------
                                           Name: Anita Dallago
                                           Title: Financial Services Officer



                                           ROGER H. MOLVAR, as
                                           Administrative Trustee



                                           By:    /s/ Roger H. Molvar
                                              ----------------------------------

                                       3