<PAGE>
================================================================================


                UNITED STATES SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                    FORM 10-Q

 [X]     QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
         EXCHANGE ACT OF 1934

         FOR THE QUARTERLY PERIOD ENDED:  MARCH 31, 2003

                                       OR

 [ ]     TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
         EXCHANGE ACT OF 1934

         FOR THE TRANSITION PERIOD FROM ____________ TO ______________

                        COMMISSION FILE NUMBER: 33-42337

                       CAPSTEAD SECURITIES CORPORATION IV
             (Exact name of Registrant as specified in its Charter)

                       DELAWARE                                 75-2390594
            (State or other jurisdiction of                  (I.R.S. Employer
            incorporation or organization)                   Identification No.)

8401 NORTH CENTRAL EXPRESSWAY, SUITE 800, DALLAS, TX               75225
       (Address of principal executive offices)                  (Zip Code)

       Registrant's telephone number, including area code: (214) 874-2323

The Registrant meets the conditions set forth in General Instruction H(1)(a) and
(b) for Form 10-Q and is therefore filing this Form under the reduced disclosure
format.

Indicate by check mark whether the Registrant (1) has filed all documents and
reports required to be filed by Section 13 or 15(d) of the Securities Exchange
Act of 1934 during the preceding 12 months (or for such shorter period that the
Registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days. YES [X] NO [ ]

                      APPLICABLE ONLY TO CORPORATE ISSUERS:

Indicate the number of shares outstanding of each of the issuer's classes of
common stock, as of the last practicable date.

Common Stock ($1.00 par value)                           1,000 as of May 1, 2003


================================================================================
<PAGE>


                       CAPSTEAD SECURITIES CORPORATION IV
                                    FORM 10-Q
                      FOR THE QUARTER ENDED MARCH 31, 2003


                                      INDEX



<Table>
<Caption>
                                                                           PAGE
                                                                           ----
<S>                                                                        <C>
                        PART I. -- FINANCIAL INFORMATION

ITEM 1.       Financial Statements

   Balance Sheets -- March 31, 2003 and December 31, 2002.................   1

   Statements of Operations -- Quarter Ended March 31, 2003 and 2002......   2

   Statements of Cash Flows -- Quarter Ended March 31, 2003 and 2002......   3

   Notes to Financial Statements..........................................   4

ITEM 2.       Management's Discussion and Analysis of
                Financial Condition and Results of Operations.............   5

ITEM 3.       Qualitative and Quantitative Disclosure of Market Risk......   6

ITEM 4.       Controls and Procedures.....................................   6


                          PART II. -- OTHER INFORMATION

ITEM 6.       Exhibits and Reports on Form 8-K............................   6

SIGNATURES................................................................   6

CERTIFICATION PURSUANT TO SECTION 302(a) OF THE
     SARBANES-OXLEY ACT OF 2002...........................................   7
</Table>

<PAGE>
                        PART I. -- FINANCIAL INFORMATION
                       CAPSTEAD SECURITIES CORPORATION IV
                                 BALANCE SHEETS
                      (IN THOUSANDS, EXCEPT PER SHARE DATA)


ITEM 1. FINANCIAL STATEMENTS


<Table>
<Caption>
                                                                      MARCH 31, 2003              DECEMBER 31, 2002
                                                                      --------------              -----------------
                                                                       (UNAUDITED)
<S>                                                                   <C>                         <C>
ASSETS
   Mortgage securities collateral                                        $36,827                      $42,256
   Cash and cash equivalents                                                  14                           14
                                                                         -------                      -------

                                                                         $36,841                      $42,270
                                                                         =======                      =======

LIABILITIES
   Collateralized mortgage securities                                    $36,363                      $41,724
                                                                         -------                      -------


STOCKHOLDER'S EQUITY
   Common stock - $1.00 par value,
     1 shares authorized,
     issued and outstanding                                                    1                            1
   Paid-in capital                                                            33                           30
   Undistributed loss                                                        (15)                         (12)
   Accumulated other comprehensive
     income                                                                  459                          527
                                                                         -------                      -------

                                                                             478                          546
                                                                         -------                      -------

                                                                         $36,841                      $42,270
                                                                         =======                      =======
</Table>


See accompanying notes to financial statements.


                                      -1-
<PAGE>

                       CAPSTEAD SECURITIES CORPORATION IV
                            STATEMENTS OF OPERATIONS
                                   (UNAUDITED)
                                 (IN THOUSANDS)


<Table>
<Caption>
                                                                                      QUARTER ENDED MARCH 31
                                                                                   ------------------------------
                                                                                    2003                   2002
                                                                                   ------                 -------
<S>                                                                                <C>                    <C>
Interest income:
   Mortgage securities collateral                                                  $  537                  $  958
   Payable to parent                                                                   --                      (1)
                                                                                   ------                  ------

       Total interest income                                                          537                     957

Interest expense on collateralized
   mortgage securities                                                                504                     906
                                                                                   ------                  ------

         Net interest income                                                           33                      51
                                                                                   ------                  ------

Other operating expenses:
   Management fees                                                                      3                       3
   Pool insurance                                                                      33                      52
                                                                                   ------                  ------

       Total other operating expenses                                                  36                      55
                                                                                   ------                  ------

Net loss                                                                               (3)                     (4)

Other comprehensive loss                                                              (68)                   (157)
                                                                                   ------                  ------

Comprehensive loss                                                                 $  (71)                 $ (161)
                                                                                   ======                  ======
</Table>


See accompanying notes to financial statements.


                                      -2-
<PAGE>

                       CAPSTEAD SECURITIES CORPORATION IV
                            STATEMENTS OF CASH FLOWS
                                   (UNAUDITED)
                                 (IN THOUSANDS)


<Table>
<Caption>
                                                                                      QUARTER ENDED MARCH 31
                                                                                   -----------------------------
                                                                                    2003                  2002
                                                                                   -------              --------
<S>                                                                                 <C>                 <C>
OPERATING ACTIVITIES:
   Net loss                                                                        $    (3)             $     (4)
   Noncash item - amortization of
     discount and premium                                                              (35)                  (79)
                                                                                   -------              --------

       Net cash used in operating activities                                           (38)                  (83)
                                                                                   -------              --------

INVESTING ACTIVITIES:
   Mortgage securities collateral:
     Principal collections on collateral                                             5,212                12,105
     Decrease in accrued interest receivable                                            35                    80
                                                                                   -------              --------

       Net cash provided by investing activities                                     5,247                12,185
                                                                                   -------              --------

FINANCING ACTIVITIES:
   Collateralized mortgage securities:
     Principal payments on securities                                               (5,212)              (12,105)
     Decrease in accrued interest payable                                               --                    (1)
   Capital contributions                                                                 3                     4
                                                                                   -------              --------

       Net cash used in financing activities                                        (5,209)              (12,102)
                                                                                   -------              --------

Net change in cash and cash equivalents                                                 --                    --

Cash and cash equivalents at beginning
   of period                                                                            14                    14
                                                                                   -------              --------
Cash and cash equivalents at end of
   period                                                                          $    14              $     14
                                                                                   =======              ========
</Table>


See accompanying notes to financial statements.


                                      -3-
<PAGE>

                       CAPSTEAD SECURITIES CORPORATION IV
                          NOTES TO FINANCIAL STATEMENTS
                                 MARCH 31, 2003
                                   (UNAUDITED)


NOTE A -- BASIS OF PRESENTATION

The accompanying unaudited financial statements have been prepared in accordance
with generally accepted accounting principles for interim financial information
and with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X.
Accordingly, they do not include all of the information and footnotes required
by generally accepted accounting principles for complete financial statements.
In the opinion of management, all adjustments (consisting of normal recurring
accruals) considered necessary for a fair presentation have been included.
Operating results for the quarter ended March 31, 2003 are not necessarily
indicative of the results that may be expected for the calendar year ending
December 31, 2003. For further information refer to the financial statements and
footnotes thereto included in the Capstead Securities Corporation IV (the
"Company") Annual Report on Form 10-K for the year ended December 31, 2002.

NOTE B -- DISCLOSURES REGARDING FAIR VALUES OF FINANCIAL INSTRUMENTS

The estimated fair values of mortgage securities collateral have been determined
using available market information and appropriate valuation methodologies.
However, considerable judgment is required in interpreting market data to
develop these estimates. In addition, fair values fluctuate on a daily basis.
Accordingly, estimates presented herein are not necessarily indicative of the
amounts that could be realized in a current market exchange. The use of
different market assumptions and/or estimation methodologies may have a material
effect on estimated fair value amounts.

Fair values are estimated using quoted market prices, when available, including
quotes made by Capstead Mortgage Corporation's lenders in connection with
designating collateral for repurchase arrangements. The following summarizes
fair value disclosures for mortgage securities collateral held
available-for-sale for the dates indicated (in thousands):

<Table>
<Caption>
                                                            MARCH 31, 2003                DECEMBER 31, 2002
                                                            --------------                -----------------
<S>                                                         <C>                           <C>
         Basis                                                $36,368                        $41,729
         Unrealized gains                                         459                            526
                                                              -------                        -------
           Fair value                                         $36,827                        $42,256
                                                              =======                        =======
</Table>

The maturity of mortgage securities collateral is directly affected by the rate
of principal prepayments by mortgagors. In addition, upon redemption of
remaining bonds outstanding pursuant to clean-up calls, released collateral may
be sold provided the collateral has paid down to within 15% of its original
issue amount. Such sales are deemed maturities under the provisions of Statement
of Financial Accounting Standards No. 115. There was no CMO collateral released
during the first quarter of 2003.


                                      -4-
<PAGE>

NOTE C -- NET INTEREST INCOME ANALYSIS

The following summarizes interest income and interest expense and the average
effective interest rates (dollars in thousands):

<Table>
<Caption>
                                                                              QUARTER ENDED MARCH 31
                                                               ----------------------------------------------------
                                                                       2003                            2002
                                                               ---------------------           --------------------
                                                                             AVERAGE                        AVERAGE
                                                               AMOUNT         RATE             AMOUNT        RATE
                                                               ------        -------           ------       -------
<S>                                                            <C>           <C>               <C>          <C>
Interest income on mortgage
   securities collateral                                       $  537         5.55%            $  958        5.97%
Interest expense on
   collateralized mortgage securities                             504         5.17                906        5.61
                                                               ------                          ------
                                                               $   33                          $   52
                                                               ======                          ======
</Table>

Changes in interest income and interest expense due to changes in interest rates
versus changes in volume were as follows (in thousands):

<Table>
<Caption>
                                                                              QUARTER ENDED MARCH 31
                                                                  ----------------------------------------------
                                                                  RATE*             VOLUME*               TOTAL
                                                                  -----             -------               ------
<S>                                                               <C>               <C>                   <C>
Interest income on mortgage
   securities collateral                                          $(64)             $ (357)               $ (421)
Interest expense on
   collateralized mortgage securities                              (66)               (336)                 (402)
                                                                  ----              ------                ------
                                                                  $  2              $  (21)               $  (19)
                                                                  ====              ======                ======
</Table>

*        The change in interest due to both volume and rate has been allocated
         to volume and rate changes in proportion to the relationship of the
         absolute dollar amounts of the change in each.


ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
        OF OPERATIONS

Capstead Securities Corporation IV (the "Company") was incorporated in Delaware
on August 16, 1991, as a special-purpose finance subsidiary of Capstead Mortgage
Corporation ("CMC") and commenced operations on December 23, 1991 with the
issuance of its first collateralized mortgage obligation ("CMO"), Series
1991-VIII. As of March 31, 2003, the Company had issued 19 series of CMOs with
an aggregate initial principal balance of $4,572,644,000. The Company has not
issued any CMOs since September 30, 1998. Only CMO Series 1998-3 remains
outstanding at March 31, 2003.

The Company's net losses are due to operational costs incurred (management and
administrative fees). Other comprehensive loss reflects the decline in
unrealized gain pledged CMO collateral since year-end as a result of relatively
high mortgage prepayments. As the underlying mortgages pay off, related
unrealized gain is eliminated.

LIQUIDITY AND CAPITAL RESOURCES

All ongoing cash CMO expenses of the Company are paid out of the excess cash
flow on the CMO issued before the residual holders receive their residual
interest. The Company believes that the excess cash flows will be sufficient to
pay ongoing CMO expenses. Cash flow requirements due to ongoing operational
costs are funded by CMC.


                                      -5-
<PAGE>

ITEM 3. QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK

The Company did not retain any economic interest in its last remaining CMO
outstanding at March 31, 2003; therefore, the Company has no exposure to
interest rate risk.

ITEM 4. CONTROLS AND PROCEDURES

As of March 31, 2003, an evaluation was performed under the supervision and with
the participation of the Company's management, including the Chief Executive
Officer ("CEO") and Senior Vice President - Control ("CFO"), of the
effectiveness of the design and operation of the Company's disclosure controls
and procedures. Based on that evaluation, the Company's management, including
the CEO and CFO, concluded that the Company's disclosure controls and procedures
were effective as of March 31, 2003. There have been no significant changes in
the Company's internal controls or in other factors that could significantly
affect internal controls subsequent to March 31, 2003.


                          PART II. -- OTHER INFORMATION

ITEM 6. EXHIBITS AND REPORTS ON FORM 8-K:

(a)    Exhibits:  The following Exhibit is presented herewith:

       Exhibit 99.1 - Certification pursuant to Section 906 of the
       Sarbanes-Oxley Act of 2002.

(b)    Reports on Form 8-K:  None



                                   SIGNATURES


Pursuant to the requirements of the Securities Exchange Act of 1934, the
Registrant has duly caused this report to be signed on its behalf by the
undersigned thereunto duly authorized.

                                        CAPSTEAD SECURITIES CORPORATION IV


Date:  May 5, 2003                By:   /s/ ANDREW F. JACOBS
                                        ----------------------------------------
                                        Andrew F. Jacobs
                                        Chairman, Chief Executive Officer and
                                        President


Date:  May 5, 2003                By:   /s/ PHILLIP A. REINSCH
                                        ----------------------------------------
                                        Phillip A. Reinsch
                                        Senior Vice President - Control



                                      -6-
<PAGE>

                       CAPSTEAD SECURITIES CORPORATION IV
                                  CERTIFICATION
                        PURSUANT TO SECTION 302(a) OF THE
                           SARBANES-OXLEY ACT OF 2002



I, Andrew F. Jacobs, certify that:

1.   I have reviewed this Quarterly Report on Form 10-Q of Capstead Securities
     Corporation IV;

2.   Based on my knowledge, this Quarterly Report does not contain any untrue
     statement of a material fact or omit to state a material fact necessary to
     make the statements made, in light of the circumstances under which such
     statements were made, not misleading with respect to the period covered by
     this Quarterly Report;

3.   Based on my knowledge, the financial statements, and other financial
     information included in this Quarterly Report, fairly present in all
     material respects the financial condition, results of operations and cash
     flows of the registrant as of, and for, the periods presented in this
     Quarterly Report;

4.   The registrant's other certifying officers and I are responsible for
     establishing and maintaining disclosure controls and procedures (as defined
     in Exchange Act Rules 13a-14 and 15d-14) for the registrant and we have:

     a)   designed such disclosure controls and procedures to ensure that
          material information relating to the registrant, including its
          consolidated subsidiaries, is made known to us by others within those
          entities, particularly during the period in which this Quarterly
          Report is being prepared;

     b)   evaluated the effectiveness of the registrant's disclosure controls
          and procedures as of a date within 90 days prior to the filing date of
          this Quarterly Report (the "Evaluation Date"); and

     c)   presented in this Quarterly Report our conclusions about the
          effectiveness of the disclosure controls and procedures based on our
          evaluation as of the Evaluation Date;

5.   The registrant's other certifying officers and I have disclosed, based on
     our most recent evaluation, to the registrant's auditors and the audit
     committee of registrant's board of directors:

     a)   all significant deficiencies in the design or operation of internal
          controls which could adversely affect the registrant's ability to
          record, process, summarize and report financial data and have
          identified for the registrant's auditors any material weaknesses in
          internal controls; and

     b)   any fraud, whether or not material, that involves management or other
          employees who have a significant role in the registrant's internal
          controls; and

6.   The registrant's other certifying officers and I have indicated in this
     Quarterly Report whether or not there were significant changes in internal
     controls or in other factors that could significantly affect internal
     controls subsequent to the date of our most recent evaluation, including
     any corrective actions with regard to significant deficiencies and material
     weaknesses.



Date:  May 5, 2003                      By:  /s/ ANDREW F. JACOBS
                                             -----------------------------------
                                             Andrew F. Jacobs
                                             Chairman, Chief Executive Officer
                                             and President


                                      -7-
<PAGE>

                       CAPSTEAD SECURITIES CORPORATION IV
                                  CERTIFICATION
                        PURSUANT TO SECTION 302(a) OF THE
                           SARBANES-OXLEY ACT OF 2002



I, Phillip A. Reinsch, certify that:

1.   I have reviewed this Quarterly Report on Form 10-Q of Capstead Securities
     Corporation IV;

2.   Based on my knowledge, this Quarterly Report does not contain any untrue
     statement of a material fact or omit to state a material fact necessary to
     make the statements made, in light of the circumstances under which such
     statements were made, not misleading with respect to the period covered by
     this Quarterly Report;

3.   Based on my knowledge, the financial statements, and other financial
     information included in this Quarterly Report, fairly present in all
     material respects the financial condition, results of operations and cash
     flows of the registrant as of, and for, the periods presented in this
     Quarterly Report;

4.   The registrant's other certifying officers and I are responsible for
     establishing and maintaining disclosure controls and procedures (as defined
     in Exchange Act Rules 13a-14 and 15d-14) for the registrant and we have:

     a)   designed such disclosure controls and procedures to ensure that
          material information relating to the registrant, including its
          consolidated subsidiaries, is made known to us by others within those
          entities, particularly during the period in which this Quarterly
          Report is being prepared;

     b)   evaluated the effectiveness of the registrant's disclosure controls
          and procedures as of a date within 90 days prior to the filing date of
          this Quarterly Report (the "Evaluation Date"); and

     c)   presented in this Quarterly Report our conclusions about the
          effectiveness of the disclosure controls and procedures based on our
          evaluation as of the Evaluation Date;

5.   The registrant's other certifying officers and I have disclosed, based on
     our most recent evaluation, to the registrant's auditors and the audit
     committee of registrant's board of directors:

     a)   all significant deficiencies in the design or operation of internal
          controls which could adversely affect the registrant's ability to
          record, process, summarize and report financial data and have
          identified for the registrant's auditors any material weaknesses in
          internal controls; and

     b)   any fraud, whether or not material, that involves management or other
          employees who have a significant role in the registrant's internal
          controls; and

6.   The registrant's other certifying officers and I have indicated in this
     Quarterly Report whether or not there were significant changes in internal
     controls or in other factors that could significantly affect internal
     controls subsequent to the date of our most recent evaluation, including
     any corrective actions with regard to significant deficiencies and material
     weaknesses.



Date:  May 5, 2003                     By:  /s/ PHILLIP A. REINSCH
                                            ------------------------------------
                                            Phillip A. Reinsch
                                            Senior Vice President - Control


                                      -8-
<PAGE>
                               INDEX TO EXHIBITS

<Table>
<Caption>
EXHIBIT
  NO.                                        DESCRIPTION
-------                                      -----------
<S>          <C>
  99.1       Certification pursuant to Section 906 of the Sarbanes-Oxley Act of 2002.
</Table>