<PAGE>

                                                                   Exhibit 10.20


                                                                  EXECUTION COPY

                        AMENDMENT No.4 AND WAIVER dated as of October 19, 2000
                  (this "Amendment"), to the Credit Agreement dated as of June
                  23, 1999, as amended by Amendment No. 1 dated as of January
                  13, 2000, Amendment No. 2 dated as of March 29, 2000 and
                  Amendment No. 3 dated as of June 30, 2000 (the "Credit
                  Agreement"), among ANTEON CORPORATION, a Virginia corporation
                  (the "Borrower"), the Lenders (as defined in Article I of the
                  Credit Agreement), CREDIT SUISSE FIRST BOSTON, a bank
                  organized under the laws of Switzerland, acting through its
                  New York branch, as an Issuing Bank (as defined in Article I
                  of the Credit Agreement), and as administrative agent (in such
                  capacity, the "Administrative Agent") for the Lenders, MELLON
                  BANK, N.A., a national banking association, as syndication
                  agent (in such capacity, the "Syndication Agent"), as
                  swingline lender (in such capacity, the "Swingline Lender"),
                  and as collateral agent (in such capacity, the "Collateral
                  Agent") for the Lenders, and DEUTSCHE BANK, AG, New York
                  Branch, as documentation agent (in such capacity, the
                  "Documentation Agent").

      A. Pursuant to the Credit Agreement, the Lenders and the Issuing Banks
have extended, and have agreed to extend, credit to the Borrower.

      B. The Borrower has informed the Administrative Agent that it intends to
purchase all of the outstanding Equity Interests of Sherikon, Inc., a Louisiana
corporation (the "Sherikon Acquisition"), pursuant to a stock purchase agreement
that is to be executed on or before October 31, 2000 (the "Acquisition Closing
Date"). As consideration for the Sherikon Acquisition, the Borrower will pay (i)
$20,800,000 in cash, and (ii) a total notional principal amount of $7,500,000 in
the form of two subordinated promissory notes. The first promissory note, in a
notional principal amount of $5,000,000, will have a repayment date of the first
anniversary of the Acquisition Closing Date (the "First Seller Note") and the
second promissory note, in a notional principal amount of $2,500,000, will have
a repayment date of the second anniversary of the Acquisition Closing Date (the
"Second Seller Note" and, together with the First Seller Note, the "Seller
Notes").

      C. The Borrower has requested that a certain provision of the Credit
Agreement be waived as set forth herein and that certain provisions of the
Credit Agreement be amended as set forth herein. The Required Lenders are
willing to grant such waiver and to amend the Credit Agreement on the terms and
subject to the conditions set forth herein.

      D. Capitalized terms used but not defined herein shall have the meanings
assigned to them in the Credit Agreement.

      Accordingly, in consideration of the mutual agreements herein contained
and other good and valuable consideration, the sufficiency and receipt of which
are hereby acknowledged, the parties hereto agree as follows:

      SECTION 1. Waiver and Agreement. The Required Lenders hereby waive
compliance by the Borrower with the provisions of Section 6.01 of the Credit
Agreement to the extent, but only to the extent, necessary to allow the issuance
of the Seller Notes,
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                                                                               2


subject to the conditions following in this Section 1. The Indebtedness basket
in Section 6.01(i) shall be unavailable until the First Seller Note is repaid in
full or otherwise discharged. The Indebtedness basket in Section 6.01(h) shall
be unavailable until the First Seller Note has been repaid in full or otherwise
discharged, at which point the Borrower may incur Indebtedness under that
section up to an aggregate amount of $2,500,000. The Indebtedness basket in
Section 6.01(h) will be completely available after both Seller Notes have been
repaid in full or otherwise discharged. The Borrower also agrees that until both
Seller Notes are repaid in full or otherwise discharged, the Borrower shall not
be permitted to incur Indebtedness in reliance on Section 6.01(g) in an
aggregate principal amount exceeding $6,500,000 at any time outstanding.

      SECTION 2. Amendments.

            (a) Section 6.01(c) of the Credit Agreement is hereby amended by
      deleting "$7,500,000" and replacing it with "$2,000,000".

            (b) Section 6.01 of the Credit Agreement is hereby amended as
      follows:

                  (i) by deleting the word "and" at the end of subsection (j);

                  (ii) by replacing the period at the end of subsection (k) with
            the words "; and"; and

                  (iii) by inserting the following new subsection (1):

                        "(l) unsecured Indebtedness of the Borrower incurred in
                  connection with a Permitted Acquisition subordinated to the
                  Obligations on terms and conditions no less favorable to the
                  Lenders than those contained in Exhibit K in an aggregate
                  principal amount not to exceed $5,500,000 at any time
                  outstanding; provided that notwithstanding Section 1(b) of
                  Exhibit K, Indebtedness incurred pursuant to this paragraph
                  (1) may have a stated maturity date and, so long as no Default
                  or Event of Default has occurred and is continuing or would
                  result therefrom, such Indebtedness may be paid or prepaid, on
                  a date that is prior to the Term Loan Maturity Date."

      SECTION 3. Representations and Warranties. To induce the other parties
hereto to enter into this Amendment, the Borrower represents and warrants to
each of the Lenders, the Administrative Agent, the Issuing Banks, the Collateral
Agent, the Syndication Agent and the Documentation Agent that, after giving
effect to this Amendment, (a) the Sherikon Acquisition is a Permitted
Acquisition under the Credit Agreement, (b) the representations and warranties
set forth in Article III of the Credit Agreement are true and correct in all
material respects on and as of the date hereof, except to the extent such
representations and warranties expressly relate to an earlier date, and (c) no
Default or Event of Default has occurred and is continuing.

      SECTION 4. Amendment Fee. The Company agrees to pay to each Lender that
executes and delivers a copy of this Amendment to the Administrative Agent (or
its counsel) at or prior to 5:00 p.m. on October 19, 2000, an amendment fee (the
"Amendment Fee") in an amount equal to 0.10% of such Lender's Revolving
Commitment (whether used or unused) and outstanding Term Loans, in each case as
of
<PAGE>
                                                                               3


the Amendment Effective Date (as defined below). The Amendment Fee shall be
payable in immediately available funds on the Amendment Effective Date. Once
paid, the Amendment Fee shall not be refundable.

      SECTION 5. Effectiveness. This Amendment shall become effective as of the
date set forth above on the date (the "Amendment Effective Date") that the
following conditions are satisfied:

            (a) the Administrative Agent shall have received the Amendment Fee;
      and

            (b) the Administrative Agent shall have received counterparts of
      this Amendment that, when taken together, bear the signatures of the
      Borrower, the Guarantors and the Required Lenders.

      SECTION 6. Effect of Amendment. Except as set forth herein, this Amendment
shall not by implication or otherwise limit, impair, constitute a waiver of, or
otherwise affect the rights and remedies of the Lenders, the Issuing Banks, the
Collateral Agent, the Administrative Agent, the Syndication Agent or the
Documentation Agent under the Credit Agreement or any other Loan Document, and
shall not alter, modify, amend or in any way affect any of the terms,
conditions, obligations, covenants or agreements contained in the Credit
Agreement or any other Loan Document, all of which are ratified and affirmed in
all respects and shall continue in full force and effect. Nothing herein shall
be deemed to entitle any Loan Party to a consent to, or a waiver, amendment,
modification other change of, any of the terms, conditions, obligations,
covenants or agreements contained in the Credit Agreement or any other Loan
Document in similar or different circumstances. This Amendment shall apply and
be effective only with respect to the provisions of the Credit Agreement
specifically referred to herein. After the date hereof, any reference to the
Credit Agreement shall mean the Credit Agreement, as modified hereby. This
Amendment shall constitute a "Loan Document" for all purposes of the Credit
Agreement and the other Loan Documents.

      SECTION 7. Counterparts. This Amendment may be executed in any number of
counterparts and by different parties hereto in separate counterparts, each of
which when so executed and delivered shall be deemed an original, but all such
counterparts together shall constitute but one and the same contract. Delivery
of an executed counterpart of a signature page of this Amendment by facsimile
transmission shall be as effective as delivery of a manually executed
counterpart hereof.

      SECTION 8. Applicable Law. THIS AMENDMENT SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

      SECTION 9. Headings. The headings of this Amendment are for purposes of
reference only and shall not limit or otherwise affect the meaning hereof.

      SECTION 10. Expenses. The Borrower agrees to reimburse the Administrative
Agent for all out-of-pocket expenses in connection with this Amendment,
including the reasonable fees, charges and disbursements of Cravath, Swaine &
Moore, counsel for the Administrative Agent.

      SECTION 11. Acknowledgement of Guarantors. Each of the Guarantors hereby
acknowledges receipt and notice of, and consents to the terms of, this
Amendment.
<PAGE>
                                                                               4


            IN WITNESS WHEREOF, all parties hereto have caused this Amendment to
be duly executed by their duly authorized officers, all as of the date and year
first above written.

                                        ANTEON CORPORATION,

                                        by:

                                            /s/ Carlton B. Crenshaw
                                           -------------------------------------
                                           Name:  Carlton B. Crenshaw
                                           Title: Sr. V.P. Finance and CFO


                                        ANALYSIS & TECHNOLOGY, INC.

                                        by:

                                            /s/ Carlton B. Crenshaw
                                           -------------------------------------
                                           Name:  Carlton B. Crenshaw
                                           Title: Vice President


                                        INTERACTIVE MEDIA CORP.,

                                        by:

                                            /s/ Carlton B. Crenshaw
                                           -------------------------------------
                                           Name:  Carlton B. Crenshaw
                                           Title: Vice President


                                        TECHMATICS, INC.,

                                        by:

                                            /s/ Carlton B. Crenshaw
                                           -------------------------------------
                                           Name:  Carlton B. Crenshaw
                                           Title: V.P. and Treasurer


                                        VECTOR DATA SYSTEMS, INC.,

                                        by:

                                            /s/ Carlton B. Crenshaw
                                           -------------------------------------
                                           Name:  Carlton B. Crenshaw
                                           Title: Treasurer
<PAGE>

                                        ANTEON-CITI LLC.,

                                        by:
                                            /s/ Carlton B. Crenshaw
                                           -------------------------------------
                                           Name:  Carlton B. Crenshaw
                                           Title: Vice President
<PAGE>
                                                                               5


                                        CREDIT SUISSE FIRST BOSTON, individually
                                        and as Administrative Agent and Issuing
                                        Bank,

                                        by:

                                            /s/ Robert Hetu
                                           -------------------------------------
                                           Name:  ROBERT HETU
                                           Title: VICE PRESIDENT

                                        by:

                                            /s/ Lalita Advani
                                           -------------------------------------
                                           Name:  LALITA ADVANI
                                           Title: ASSISTANT VICE PRESIDENT


                                        MELLON BANK, N.A., individually and as
                                        Collateral Agent, Swingline Lender and
                                        Syndication Agent,

                                        by:

                                           -------------------------------------
                                           Name:
                                           Title:
<PAGE>
                                                                               5


                                        CREDIT SUISSE FIRST BOSTON, individually
                                        and as Administrative Agent and Issuing
                                        Bank,

                                        by:

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

                                        by:

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


                                        MELLON BANK, N.A., individually and as
                                        Collateral Agent, Swingline Lender and
                                        Syndication Agent,

                                        by:

                                            /s/ David H Reed
                                           -------------------------------------
                                           Name: David H. Reed
                                           Title: 1st Vice President
<PAGE>

                                                         Signature Page to First
                                                             Amendment to Bergen
                                                      Bruswig Corporation Credit
                                                                       Agreement


                                        Name of Institution:

                                        AmSouth Bank
                                        ----------------------------------------


                                        by:

                                            /s/ Stephen V. Mangiante
                                        ----------------------------------------
                                        Name: STEPHEN V. MANGIANTE
                                        Title: ATTORNEY-IN-FACT
<PAGE>

                                                               Signature Page to
                                                           Amendment dated as of
                                                      October 19, 2000 to Anteon
                                                              Corporation Credit
                                                                       Agreement


                                        Name of Insititution:

                                        Bank Polska Kasa Opiski S.A.,
                                        ----------------------------------------
                                        New York Branch


                                        by:

                                            /s/ Hussein B. El-Tawil
                                        ----------------------------------------
                                        Name: Hussein B. El-Tawil
                                        Title: Vice President

<PAGE>

                                                               Signature Page to
                                                           Amendment dated as of
                                                      October 19, 2000 to Anteon
                                                              Corporation Credit
                                                                       Agreement


                                        Name of Insititution:

                                        Branch Banking and Trust Company
                                          of Virginia
                                        ----------------------------------------


                                        by

                                           /s/ William A. Nalls
                                        ----------------------------------------
                                        Name: William A. Nalls
                                        Title: Senior Vice President

<PAGE>

                                                               Signature Page to
                                                           Amendment dated as of
                                                      October 19, 2000 to Anteon
                                                              Corporation Credit
                                                                       Agreement


                                        Name of Insititution:

                                        DEUTSCHE BANK
                                        ----------------------------------------


                                        by

                                           /s/ Paddy Dowling
                                        ----------------------------------------
                                        Name: Paddy Dowling
                                        Title: Vice President

<PAGE>

                                                               Signature Page to
                                                           Amendment dated as of
                                                      October 19, 2000 to Anteon
                                                              Corporation Credit
                                                                       Agreement


                                        Name of Insititution:

                                        FLEET NATIONAL BANK
                                        BOSTON, MASSACHUSETTS
                                        ----------------------------------------


                                        by

                                            /s/ Roger C. Boucher
                                        ----------------------------------------
                                        Name: ROGER C. BOUCHER
                                        Title: DIRECTOR
<PAGE>

                                                               Signature Page to
                                                           Amendment dated as of
                                                      October 19, 2000 to Anteon
                                                              Corporation Credit
                                                                       Agreement


                                        Name of Insititution:

                                        IBM CREDIT CORPORATION
                                        ----------------------------------------


                                        by

                                           /s/ Thomas S. Curcio
                                        ----------------------------------------
                                        Name: Thomas S. Curcio
                                        Title: Manager of Credit, Commercial
                                               & Specialty Financing
<PAGE>

                                                               Signature Page to
                                                           Amendment dated as of
                                                      October 19, 2000 to Anteon
                                                              Corporation Credit
                                                                       Agreement


                                        Name of Insititution:

                                        THE BANK OF NOVA SCOTIA
                                        ----------------------------------------


                                        by

                                           /s/ John Hopmans
                                        ----------------------------------------
                                        Name: JOHN HOPMANS
                                        Title: MANAGING DIRECTOR

<PAGE>

                                                               Signature Page to
                                                           Amendment dated as of
                                                      October 19, 2000 to Anteon
                                                              Corporation Credit
                                                                       Agreement


                                        Name of Insititution:

                                        PNC BANK, NA
                                        ----------------------------------------


                                        by

                                           /s/ Daniel J. Paull
                                        ----------------------------------------
                                        Name: Daniel J. Paull
                                        Title: Vice President

<PAGE>

                                                               Signature Page to
                                                           Amendment dated as of
                                                      October 19, 2000 to Anteon
                                                              Corporation Credit
                                                                       Agreement


                                        Name of Insititution:

                                        TRANSAMERCIA BUSINESS CREDIT CORPORATION
                                        ----------------------------------------


                                        by

                                           /s/ Steve Goetschius
                                        ----------------------------------------
                                        Name: Steve Goetschius
                                        Title: SVP
<PAGE>

                        FINANCIAL OFFICER'S CERTIFICATE

      Pursuant to the terms and conditions of the Credit Agreement dated as of
June 23, 1999 (as amended, supplemented or otherwise modified from time to time,
the "Credit Agreement"), among Anteon Corporation (the "Borrower"), the lenders
from time to time party thereto (the "Lenders"), Credit Suisse First Boston, as
administrative agent for the Lenders (in such capacity, the "Administrative
Agent"), and as issuing bank, Mellon Bank, N.A., as collateral agent (in such
capacity, the "Collateral Agent"), swingline lender and syndication agent, and
Deutsche Bank AG, New York Branch, as documentation agent, Borrower hereby
delivers this Financial Officer's Certificate to the Administrative Agent
pursuant to Section 6.04 (i) of the Credit Agreement. Capitalized terms used but
not defined herein shall have the meanings attributed to such terms in the
Credit Agreement.

      The undersigned, on behalf of the Borrower, hereby certifies and warrants
that:

      (a)   Carlton B. Crenshaw is a Financial officer of the Borrower and that,
            as such, he is authorized to execute this certificate for and on
            behalf of the Borrower.

      (b)   The Borrower intends to purchase all of the outstanding Equity
            Interests of Sherikon, Inc., pursuant to a stock purchase agreement
            to be executed on or before October 31, 2000.

      (c)   This certificate is provided to evidence the Borrower's compliance
            with the requirements of Section 6.04(i) in connection with the
            Permitted Acquisition described in paragraph (b) above.

      (d)   The pro forma calculations required to be made pursuant to Section
            6.04(i) were made in accordance with the requirements set forth
            therein and are hereby certified as having been prepared in good
            faith based upon reasonable assumptions.

      IN WITNESS WHEREOF, the undersigned has executed and delivered this
certificate, this 19th day of October, 2000.


                                        ANTEON CORPORATION


                                        by: /s/ Carlton B. Crenshaw
                                           -------------------------------------
                                           Name:  Carlton B. Crenshaw
                                           Title: Financial Officer