<PAGE>

                                                                 EXHIBIT 10.30.4

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]

                                August 25, 2004

Oak Street Mortgage, LLC
11595 N. Meridian Street
Suite 400
Carmel, IN 46032

      Re: Engagement Letter

Dear Sirs

      Friedman, Billings, Ramsey & Co., Inc. ("FBR") would be pleased to act as
lead underwriter and sole book runner for Oak Street Mortgage, LLC and its
affiliates (the "Company") subject to the terms and conditions of this letter
agreement (the "Agreement"). Reference is made to a repurchase program either
between CDC Mortgage Capital, Inc. ("CDC") for which FBR is providing a
guarantee of certain obligations of the Company and the Company or its
affiliate, which is currently being negotiated or between FBR and the Company or
its affiliate (collectively, the "Repurchase Program") or any similar
transaction (each a "Transaction").

      1. The Offering.

      1(a) During the Engagement Period (as defined in Section 6, below), FBR
will be the lead underwriter and sole book runner for the Company with respect
to any proposed offerings (collectively, the "Offering") of mortgage-backed
securities (the "Securities") representing an interest in, or secured by,
mortgage loans originated or acquired by the Company or any of its affiliates
that become a participant in the Repurchase Program. The actual terms of the
Offering will depend on the outcome of FBR's due diligence investigation and
market conditions, and will be subject to negotiation between the Company and
FBR.

      1(b) In undertaking its role as lead underwriter and sole book runner, FBR
anticipates that its activities would include the following, as requested from
time to time by the Company:

      (i)   Reviewing the proposed transaction;

      (ii)  Assisting the Company in the Company's determination of appropriate
            structure;

      (iii) Conducting an examination of documents and records pertaining to the
            Company and the related assets, interviewing Company personnel, and
            making such other reasonable investigations as FBR deems necessary
            and appropriate under the circumstances; and

                                          Friedman, Billings, Ramsey & Co., Inc.
                                                    1001 Nineteenth Street North
                                                       Arlington, Virginia 22209
                                                                    703.312.9500
                                                                     www.fbr.com

<PAGE>

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]                                                     Engagement Letter
                                                                  August 25,2004
                                                                          Page 2

      (iv)  Performing financial analysis of the Company and its assets and
            comparisons with other companies and similar transactions in its
            industry.

      1(c) FBR intends to conduct the Offering, only after execution of an
underwriting agreement, which will include customary representations and
warranties, covenants, conditions, termination provisions and indemnification,
contribution and limitation of liability, all satisfactory to FBR in its sole
discretion. FBR's willingness to execute an underwriting agreement and conduct
the Offering will be subject to its satisfaction, in its sole discretion and
judgment, with market conditions and the results of its due diligence
investigation of the Company and its business. If FBR fails to confirm its
willingness to conduct an Offering on the terms provided herein within fifteen
(15) business days of written request by the Company, the Company shall be free
to engage another firm to provide such services with respect to that Offering

      1(d) Except as contemplated in the last sentence of Section 1(c), during
the Engagement Period, the Company will not offer any Securities for sale to, or
solicit any offers to buy from, any person or persons, whether directly or
indirectly, otherwise than through FBR.

      2. Reserved.

      3. Information to be Supplied; Confidentiality.

      3(a) In connection with FBR's activities on behalf of the Company, the
Company will furnish FBR with all financial and other information regarding the
Company that FBR reasonably believes appropriate to its assignment (all such
information so furnished by the Company, whether furnished before or after the
date of this Agreement, being referred to herein as the "Information"). The
Company will provide FBR with access to the officers, directors, employees,
independent accountants, legal counsel and other advisors and consultants for
the Company. The Company recognizes and agrees that FBR (i) will use and rely
primarily on the Information and information available from generally recognized
public sources in performing the services contemplated by this Agreement without
independently verifying the Information or such other information, (ii) does not
assume responsibility for the accuracy of the Information or such other
information, and (iii) will not make an appraisal of any assets or liabilities
owned or controlled by the Company or its market competitors.

      3(b) FBR will maintain the confidentiality of the Information and, unless
and until such information shall have been made publicly available by the
Company or by others without breach of a confidentiality agreement, shall
disclose the Information only as authorized by the Company or as required by
law, including NASD Rule 2711, or by order of a governmental authority or court
of competent jurisdiction. In the event that FBR is legally required to make
disclosure of

<PAGE>

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]                                                     Engagement Letter
                                                                 August 25, 2004
                                                                          Page 3

any of the Information, FBR will give notice to the Company prior to such
disclosure, to the extent that FBR can practically do so.

The foregoing paragraph shall not apply to information that:

      (i)   at the time of disclosure by the Company is, or thereafter becomes,
            generally available to the public or within the industries in which
            the Company or FBR or its affiliates conduct business, other than as
            a direct result of a breach by FBR of its obligations under this
            Agreement;

      (ii)  prior to or at the time of disclosure by the Company, was already in
            the possession of, or conceived by, FBR or any of its affiliates, or
            could have been developed by them from information then in their
            possession, by the application of other information or techniques in
            their possession, generally available to the public, or available to
            FBR or its affiliates other than from the Company;

      (iii) at the time of disclosure by the Company or thereafter, is obtained
            by FBR or and of its affiliates from a third party who FBR
            reasonably believes to be in possession of the information not in
            violation of any contractual, legal or fiduciary obligation to the
            Company with respect to that information; or

      (iv)  is independently developed by FBR or its affiliates.

      3(c) Nothing in this Agreement shall be construed to limit the ability of
FBR or its affiliates to pursue, investigate, analyze, invest in, or engage in
investment banking, financial advisory or any other business relationships with,
entities other than the Company, notwithstanding that such entities may be
engaged in a business which is similar to or competitive with the business of
the Company, and notwithstanding that such entities may have actual or potential
operations, products, services, plans, ideas, customers or supplies similar or
identical to the Company's, or may have been identified by the Company as
potential merger or acquisition targets or potential candidates for some other
business combination, cooperation or relationship. The Company expressly
acknowledges and agrees that it does not claim any proprietary interest in the
identity of any other entity in its industry or otherwise, and that the identity
of any such entity is not confidential information.

      3(d) The Company acknowledges that all advice (written or oral) given by
FBR to the Company is intended solely for the benefit and use of the Company.
Other than to the extent required to be reflected in Board of Directors and
committee meeting minutes, no advice (written or oral) of FBR hereunder shall be
used, reproduced, disseminated, quoted or referred to at any time, in any
manner, or for any purpose, nor shall any public references to FBR be made by
the Company (or such persons), without the prior written consent of FBR.

<PAGE>

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]                                                     Engagement Letter
                                                                 August 25, 2004
                                                                          Page 4

      3(e) If the Offering is conducted as a private placement, the Company
hereby authorizes FBR to transmit to the prospective purchasers of the
Securities a Private Placement Memorandum with attached exhibits and such
supplements as may from time to time be prepared by the Company (collectively
the "Memorandum"). The Company represents and warrants that the Memorandum (i)
will be prepared by the management of the Company, (ii) will be reviewed and
approved by the management and the Board of Directors of the Company; and (iii)
will not contain any untrue statement of a material fact or omit to state a
material fact required to be stated therein or necessary to make the statements
therein or previously made, in light of the circumstances under which they were
made, not misleading. The Company will advise FBR immediately of the occurrence
of any event or any other change known to the Company which results in the
Memorandum containing an untrue statement of a material fact or omitting to
state a material fact required to be stated therein or necessary to make the
statements therein or previously made, in light of the circumstances under which
they were made, not misleading.

      3(f) The Company agrees that any documents or information received from
FBR, its affiliates or advisors in connection with this Agreement and the
transactions contemplated herein may contain information regarding the structure
of the transactions that has been developed by FBR on a proprietary basis. The
Company agrees to treat confidentially all such information received hereunder,
including the information set forth in any term sheet prepared by or on behalf
of FBR. Accordingly, such documents may not be provided to others (other than
the Company's legal counsel and accountants who are made aware of the
proprietary nature of such documents) without FBR's prior written approval,
except as required by law or legal process or if, based upon the advice of legal
counsel, disclosure is necessary under laws applicable to the Company.

      4. Fees and Expenses.

      4(a) FBR's compensation for its services in connection with each Offering
shall be determined on the basis of underwriting discounts, fees or concessions
that are equivalent to those available from third parties at the time for like
services on similar transactions. Notwithstanding the foregoing, compensation to
be paid to FBR shall be reduced: (1) with respect to the first Offering, by a
reduction of $125,000 and (2) with respect to each Offering (including the first
Offering), by an amount equal to the weighted average Price Differential accrued
(through the date two Business Days preceding the date of the related
underwriting agreement) under the Master Repurchase Agreement with CDC for the
financing of the mortgage loans that will secure or collateralize the securities
issued in such Offering, to the extent such weighted average Price Differential
actually accrued exceeds the Price Differential that would have accrued had the
Price Differential been based on a Pricing Spread of 1.00%. All reasonable and
customary out-of-pocket legal, rating agency fees, filing fees and expenses
incurred in preparation of documentation are due from the Company on the closing
date of each Offering and FBR shall have no obligation with respect to any such
reasonable and customary fees or expenses.

<PAGE>

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]                                                     Engagement Letter
                                                                 August 25, 2004
                                                                          Page 5

      4(b) In addition to the Fee payable to FBR under this Agreement, the
Company agrees to reimburse FBR upon request for its reasonable out-of-pocket
expenses incurred in connection with its services under this Agreement whether
or not the Offering is consummated, including any fees and disbursements of
FBR's legal counsel (the "Expense Reimbursement").

      4(c) The Company acknowledges and agrees that it will be responsible for
and shall pay all costs and expenses incident to the purchase, sale and delivery
of Securities in the Offering, including, without limitation, all fees and
expenses of filing with the SEC and the NASD; all Blue Sky fees and expenses;
fees and disbursements of counsel and accountants for the Company; printing
costs; costs of background investigations; all road show costs (regardless of
the form in which the roadshow is conducted) and expenses of Company and FBR
personnel, including but not limited to, commercial or charter air travel (with
first-class or charter air travel subject to advance Company approval), and
local hotel accommodations and transportation.

      5. Indemnification, Contribution, and Limitation of Liability. The Company
agrees to indemnify FBR and its controlling persons, representatives and agents
in accordance with the indemnification provisions set forth in Appendix I, and
agrees to the other provisions of Appendix I, which is incorporated herein by
this reference.

      6. Term of Engagement Period; Survival of Provisions

      6(a) The term of FBR's engagement (the "Engagement Period") shall be for a
period through the earlier of (1) six (6) months following the period during
which FBR is providing financing to the Company or its affiliates or providing a
guarantee or other enhancement to the Company's financing facilities provided by
other parties and (2) the closing of Offerings for which FBR serves as lead
underwriter and sole book runner which equal at least $1.5 billion in offered
Securities. Notwithstanding the foregoing, the Company may terminate the
Engagement Period immediately upon written notice to FBR if FBR has failed or is
failing to perform its services in connection with this engagement to the
satisfaction of the Company as determined by the Company in its sole discretion.

      6(b) This Agreement and the Appendix shall survive any termination of the
Engagement Period. With respect to the fees and expenses set forth in Section 4,
upon termination of the Engagement Period, FBR shall be entitled to collect all
such fees and expenses accrued through the date of termination.

      6(c) During the Engagement Period, the Company will not solicit or
negotiate with any other person to act as underwriter, or placement agent or to
provide other investment banking services to the Company relating to
transactions similar to the Offerings; provided, that, this Section 6(c) shall
not impair the Company's right and ability to engage (i) independent consultants
to provide financial advice, including advice respecting the structure and
economics

<PAGE>

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]                                                     Engagement Letter
                                                                 August 25, 2004
                                                                          Page 6

of securitizations or (ii) investment banking firms to provide assistance in
effectuating hedging and derivative transactions.

      7. Reserved.

      8. Reserved

      9. Reserved

      10. Independent Contractor; No Fiduciary Duty. The Company acknowledges
and agrees that it is a sophisticated business enterprise and that FBR has been
retained pursuant to this Agreement to act as financial advisor to the company
solely with respect to the matters set forth herein. In such capacity, FBR shall
act as an independent contractor, and any duties of FBR arising out of its
engagement pursuant to this Agreement shall be contractual in nature and shall
be owed solely to the Company. Each party disclaims any intention to impose any
fiduciary duty on the other.

      11. Beneficiaries. This Agreement shall inure to the sole and exclusive
benefit of FBR and the Company and the persons referred to in Appendix I and
their respective successors and representatives. The obligations and liabilities
under this Agreement shall be binding upon FBR and the Company.

      12. Governing Law. This Agreement shall be governed by and construed in
accordance with the laws of the Commonwealth of Virginia applicable to contracts
executed and to be wholly performed therein without giving effect to its
conflicts of laws principles or rules. Any dispute hereunder shall be brought in
a court in the Commonwealth of Virginia.

      13. Amendments. This Agreement may be modified or amended, or its
provisions waived, only by a writing signed by the person or persons against
whom enforcement of the modification, amendment or waiver is sought.

      14. Announcements of Offering. If an Offering is consummated in which FBR
acts as underwriter or otherwise, FBR may, at its option and expense, place an
announcement in such newspapers and periodicals as FBR may choose stating that
FBR has so acted, and the capacity in which it has acted.

      15. No Commitment. This Agreement does not and will not constitute any
agreement, commitment or undertaking, express or implied on the part of FBR or
any affiliate to purchase or to sell any securities or to provide any financing
and does not ensure the successful arrangement or completion of any Offering.

      16. Entire Agreement. This Agreement constitutes the entire Agreement
between the

<PAGE>

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]                                                     Engagement Letter
                                                                 August 25, 2004
                                                                          Page 7

parties and supersedes and cancels any and all prior or contemporaneous
arrangements, understandings and agreements, written or oral, between them
relating to the subject matter hereof. This Agreement does not affect the
agreements that are the subject of the engagement letter, dated October 14,
2003, between the parties and relating to the issuance and offering of equity
securities of Oak Street Financial Services, Inc.

      17. Severability.  If any portion of this Agreement shall be held or made
unenforceable or invalid by a statute, rule, regulation, decision of a tribunal
or otherwise, the remainder of this Agreement shall not be affected thereby and
shall remain in full force and effect, and, to the fullest extent, the
provisions of the Agreement shall be severable.

      18. Headings. The descriptive headings of the paragraphs subparagraph and
Appendix of this Agreement are inserted for convenience only, do not constitute
a part of this Agreement and shall not affect in any way the meaning or
interpretations of this Agreement.

      19. Failure or Delay No Waiver. It is understood and agreed that failure
or delay by either the Company or FBR in exercising any right, power or
privilege hereunder shall not operate as a waiver thereof, nor shall any single
or partial exercise thereof preclude any other or further exercise thereof or
the exercise of any other right, power, or privilege hereunder.

      20. Waiver of Trial by Jury. EACH OF FBR AND THE COMPANY WAIVES ALL RIGHT
TO TRIAL BY JURY IN ANY ACTION, PROCEEDING, CLAIM, OR COUNTERCLAIM (WHETHER
BASED ON CONTRACT, TORT OR OTHERWISE) RELATED TO OR ARISING OUT OF THIS
AGREEMENT.

If the foregoing terms correctly set forth our agreement, please sign and return
to us a duplicate copy of this Agreement. We look forward to working with you
toward the successful conclusion of this engagement and developing a long term
relationship with the Company.

Very truly yours,

FRIEDMAN, BILLINGS, RAMSEY & CO., INC.

By: /s/ Richard J. Hendrix
    -------------------------------
Name: Richard J. Hendrix

Title: President and Chief Operating Officer

<PAGE>

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]                                                     Engagement Letter
                                                                 August 25, 2004
                                                                          Page 8

Confirmed and accepted as of
this 25th day of August, 2004:

OAK STREET MORTGAGE, LLC

By: /s/ Craig Royal
   --------------------------------
Name: Craig Royal
     ------------------------------
Title: Chief Financial Officer
      -----------------------------
<PAGE>

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]                                                     Engagement Letter
                                                                 August 25, 2004
                                                                          Page 9

                                   APPENDIX I

      The Company agrees to indemnify and hold harmless FBR and its affiliates
(as defined in Rule 405 under the Securities Act of 1933, as amended) and their
respective directors, officers, employees, agents and controlling persons (FBR
and each such person being an "Indemnified Party") from and against all losses,
claims, damages and liabilities (or actions, including shareholder actions, in
respect thereof), joint or several, to which such Indemnified Party may become
subject under any applicable federal or state law, or otherwise, which are
related to or result from the performance by FBR of the services contemplated by
or the engagement of FBR pursuant to, this Agreement and will promptly reimburse
any Indemnified Party for all reasonable expenses (including reasonable counsel
fees and expenses) as they are incurred in connection with the investigation of,
preparation for or defense arising from any threatened or pending claim, whether
or not such Indemnified Party is a party and whether or not such claim, action
or proceeding is initiated or brought by the Company. The Company will not be
liable to any Indemnified Party under the foregoing indemnification and
reimbursement provisions, (i) for any settlement by an Indemnified Party
effected without its prior written consent (not to be unreasonably withheld); or
(ii) to the extent that any loss, claim, damage or liability is found in a
final, non-appealable judgment by a court of competent jurisdiction to have
resulted primarily from FBR's willful misconduct or gross negligence. The
Company also agrees that no Indemnified Party shall have any liability (whether
direct or indirect, in contract or tort or otherwise) to the Company or its
security holders or creditors related to or arising out of the engagement of FBR
pursuant to, or the performance by FBR of the services contemplated by, this
Agreement except to the extent that any loss, claim, damage or liability is
found in a final, non-appealable judgment by a court of competent jurisdiction
to have resulted primarily from FBR's willful misconduct or gross negligence.

      Promptly after receipt by an Indemnified Party of notice of any intention
or threat to commence an action, suit or proceeding or notice of the
commencement of any action, suit or proceeding, such Indemnified Party will, if
a claim in respect thereof is to be made against the Company pursuant hereto,
promptly notify the Company in writing of the same. In case any such action is
brought against any Indemnified Party and such Indemnified Party notifies the
Company of the commencement thereof, the Company may elect to assume the defense
thereof, with counsel reasonably satisfactory to such Indemnified Party, and an
Indemnified Party may employ counsel to participate in the defense of any such
action provided, that the employment of such counsel shall be at the Indemnified
Party's own expense, unless (i) the employment of such counsel has been
authorized in writing by the Company, (ii) the Indemnified Party has reasonably
concluded (based upon advice of counsel to the Indemnified Party) that there may
be legal

<PAGE>

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]                                                     Engagement Letter
                                                                 August 25, 2004
                                                                         Page 10

defenses available to it or other Indemnified Parties that are different from or
in addition to those available to the Company, or that a conflict or potential
conflict exists (based upon advice of counsel to the Indemnified Party) between
the Indemnified Party and the Company that makes it impossible or inadvisable
for counsel to the Indemnifying Party to conduct the defense of both the Company
and the Indemnified Party (in which case the Company will not have the right to
direct the defense of such action on behalf of the Indemnified Party), or (iii)
the Company has not in fact employed counsel reasonably satisfactory to the
Indemnified Party to assume the defense of such action within a reasonable time
after receiving notice of the action, suit or proceeding, in each of which cases
the reasonable fees, disbursements and other charges of such counsel will be at
the expense of the Company; provided, further, that in no event shall the
Company be required to pay fees and expenses for more than one firm of attorneys
representing Indemnified Parties unless the defense of one Indemnified Party is
unique or separate from that of another Indemnified Party subject to the same
claim or action. Any failure or delay by an Indemnified Party to give the notice
referred to in this paragraph shall not affect such Indemnified Party's right to
be indemnified hereunder, except to the extent that such failure or delay causes
actual harm to the Company, or prejudices its ability to defend such action,
suit or proceeding on behalf of such Indemnified Party.

      If the indemnification provided for in this Agreement is for any reason
held unenforceable by an Indemnified Party, the Company agrees to contribute to
the losses, claims, damages and liabilities for which such indemnification is
held unenforceable (i) in such proportion as is appropriate to reflect the
relative benefits to the Company, on the one hand, and FBR on the other hand, of
the Offering (or other Transaction) as contemplated whether or not the Offering
(or Other Transaction) is consummated or, (ii) if (but only if) the allocation
provided for in clause (i) is for any reason unenforceable, in such proportion
as is appropriate to reflect not only the relative benefits referred to in
clause (i) but also the relative fault of the Company, on the one hand and FBR,
on the other hand, as well as any other relevant equitable considerations. The
Company agrees that for the purposes of this paragraph the relative benefits to
the Company and FBR of the Offering (or other Transactions) as contemplated
shall be deemed to be in the same proportion that the total value received or
contemplated to be received by the Company or its shareholders, as the case may
be, as a result of or in connection with the Offering (or other Transactions)
bear to the fees paid or to be paid to FBR under this Agreement. Notwithstanding
the foregoing, the Company expressly agrees that FBR shall not be required to
contribute any amount in excess of the amount by which fees paid FBR hereunder
(excluding reimbursable expenses), exceeds the amount of any damages which FBR
has otherwise been required to pay.

      The Company agrees that without FBR's prior written consent, which shall
not be unreasonably withheld, it will not settle, compromise or consent to the
entry of any judgment in any pending or threatened claim, action or proceeding
in respect of which indemnification could be sought under the indemnification
provisions of this Agreement (in which FBR or any other Indemnified Party is an
actual or potential party to such claim, action or proceeding), unless such
settlement, compromise or consent includes an unconditional release of each
Indemnified Party from all liability arising out of such claim, action or
proceeding.

<PAGE>

                            FRIEDMAN BILLINGS RAMSEY

[FBR LOGO]                                                     Engagement Letter
                                                                 August 25, 2004
                                                                         Page 11

      In the event that an Indemnified Party is requested or required to appear
as a witness in any action brought by or on behalf of or against the Company in
which such Indemnified Party is not named as a defendant, the Company agrees to
promptly reimburse FBR on a monthly basis for all expenses incurred by it in
connection with such Indemnified Party's appearing and preparing to appear as
such a witness, including, without limitation, the reasonable fees and
disbursements of its legal counsel. In addition to any reimbursed fees, expenses
or costs outlined hereunder, FBR shall also receive from the Company cash
compensation of $2,000.00 per person, per day, plus reasonable out-of-pocket
expenses and costs should FBR be required to provide testimony in any formal or
informal proceeding regarding the Company.

      If multiple claims are brought with respect to at least one of which
indemnification is permitted under applicable law and provided for under this
Agreement, the Company agrees that any judgment or arbitration award shall be
conclusively deemed to be based on claims as to which indemnification is
permitted and provided for, except to the extent the judgment or arbitration
award expressly states that it, or any portion thereof, is based solely on a
claim as to which indemnification is not available.

Confirmed and Agreed to:

___________________________________________
(Signature on behalf of the Company)

___________________________________________
(Date)