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                                                                    EXHIBIT 10.4

                                Irrevocable Proxy

         The undersigned Stockholder of Advanced Communication Systems, Inc., a
Delaware corporation (the "Company"), hereby appoints and constitutes The Titan
Corporation, a Delaware corporation ("Acquiror"), the attorneys and proxies of
the undersigned, with full power of substitution and resubstitution, to the full
extent of the undersigned's voting rights with respect to (a) the outstanding
shares of common stock, par value $.01 per share, of the Company (the "Company
Common Stock") or any other capital stock of the Company (collectively with the
Company Common Stock, the "Capital Stock") owned of record by the undersigned as
of the date of this proxy, which shares are specified on the final page of this
proxy, and (b) any and all other shares of Capital Stock of the Company which
the undersigned may acquire on or after the date hereof, provided that this
proxy automatically shall be revoked with respect to any shares of Capital Stock
that are sold, transferred or otherwise disposed of by the undersigned in
accordance with the Company Stockholders Agreement (as hereinafter defined)
("Transferred Stock") effective as of the date of such sale, transfer or other
disposition ("Date of Disposition"). Upon the execution hereof, all prior
proxies given by the undersigned with respect to any of the Capital Stock are
hereby revoked, and the undersigned agrees that no subsequent proxy will be
given with respect to the voting of any of the Capital Stock (other than
Transferred Stock after the Date of Disposition given by the purchaser,
transferee or other beneficiary of such other disposition with respect to such
stock) except to the extent that such proxies do not prevent the voting of this
proxy in favor of the transactions described herein.

         This proxy is coupled with an interest and irrevocable. This proxy is
granted (i) in connection with the execution and delivery of the Company
Stockholders Agreement, dated as of the date hereof, among Acquiror and the
undersigned (the "Company Stockholders Agreement") and (ii) in consideration of
Acquiror entering into the Agreement and Plan of Merger, dated as of the date
hereof, among Acquiror, A T Acquisition Corp., a Delaware corporation (the
"Merger Sub") and the Company (the "Merger Agreement").

         The proxy named above (and its successors) will, prior to the
Termination Date (as hereinafter defined), be empowered, and may exercise this
proxy, to vote the Capital Stock at any meeting of the Stockholders of the
Company, however called, or in connection with any solicitation of written
consents from Stockholders of the Company, called or solicited, as the case may
be, for the purpose of voting on the Merger Agreement and the transactions
contemplated thereby in favor of the approval and adoption of the Merger
Agreement and the approval of the merger contemplated thereby, and in favor of
each of the other actions contemplated by the Merger Agreement. The undersigned
may vote the Capital Stock on all other matters.

         This proxy shall be binding upon the representatives, successors and
permitted assigns of the undersigned.



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         If any provision of this proxy or any part of any such provision is
held under any circumstances to be invalid or unenforceable in any jurisdiction,
then (a) such provision or part thereof shall, with respect to such
circumstances and in such jurisdiction, be deemed amended to conform to
applicable laws so as to be valid and enforceable to the fullest possible
extent, (b) the invalidity or unenforceability of such provision or part thereof
under such circumstances and in such jurisdiction shall not affect the validity
or enforceability of such provision or part thereof under any other
circumstances or in any other jurisdiction, and (c) the invalidity or
unenforceability of such provision or part thereof shall not affect the validity
or enforceability of the remainder of such provision or the validity or
enforceability of any other provision of this proxy. Each provision of this
proxy is separable from every other provision of this proxy, and each part of
each provision of this proxy is separable from every other part of such
provision.

         This proxy and the rights contained herein shall terminate upon the
termination of the Company Stockholders Agreement as provided therein (the
"Termination Date").

Date:  December 9, 1999

                                    /s/ Thomas A. Costello
                                    -------------------------------
                                    Thomas A. Costello

                                    /s/ Margaret M. Costello
                                    -------------------------------
                                    Margaret M. Costello


                                    Number of shares of common stock and other
                                    capital stock of the Company owned of record
                                    and beneficially as of the date of this
                                    proxy: 847,500





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