<PAGE>

                                                                    EXHIBIT 10.4

                        SIXTH LOAN MODIFICATION AGREEMENT

      This Sixth Loan Modification Agreement (this "Loan Modification
Agreement") is entered into as of March 29, 2005, by and between SILICON VALLEY
BANK, a California-chartered bank, with its principal place of business at 3003
Tasman Drive, Santa Clara, California 95054 and with a loan production office
located at One Newton Executive Park, Suite 200, 2221 Washington Street, Newton,
Massachusetts 02462 ("Bank") and SCANSOFT, INC., a Delaware corporation with
offices at 9 Centennial Drive, Peabody, Massachusetts 01960 ("Borrower").

1.    DESCRIPTION OF EXISTING INDEBTEDNESS AND OBLIGATIONS. Among other
      indebtedness and obligations which may be owing by Borrower to Bank,
      Borrower is indebted to Bank pursuant to a loan arrangement dated as of
      October 31, 2002, evidenced by, among other documents, (i) a certain Loan
      and Security Agreement dated as of October 31, 2002 between Borrower and
      Bank, as amended by a certain First Loan Modification Agreement dated May
      7, 2003, effective as of March 31, 2003, as further amended by a certain
      Second Loan Modification Agreement dated as of June 18, 2003, as further
      amended by a certain Third Loan Modification Agreement dated as of August
      11, 2003, as further amended by a certain Fourth Loan Modification
      Agreement dated as of September 30, 2003, and as further amended by a
      certain Fifth Loan Modification Agreement dated as of March 31, 2004 (as
      may be further amended from time to time, the "Loan Agreement"), and (ii)
      a certain Negative Pledge Agreement dated as of October 31, 2002 (the
      "Negative Pledge Agreement"). Capitalized terms used but not otherwise
      defined herein shall have the same meaning as in the Loan Agreement.

      Hereinafter, all indebtedness and obligations owing by Borrower to Bank
      shall be referred to as the "Obligations".

2.    DESCRIPTION OF COLLATERAL. Repayment of the Obligations is secured by the
      Collateral as described in the Loan Agreement and a certain Pledge
      Agreement dated October 31, 2002 (together with any other collateral
      security granted to Bank, the "Security Documents").

      Hereinafter, the Security Documents, together with all other documents
      evidencing or securing the Obligations shall be referred to as the
      "Existing Loan Documents".

3.    DESCRIPTION OF CHANGE IN TERMS.

            Modification to Loan Agreement. Effective as of March 29, 2005, the
            definition of "Current Liabilities" in Section 5 of the Schedule to
            the Loan Agreement, entitled "Financial Covenants", shall be amended
            as follows:

                  ""Current Liabilities" are the aggregate amount of Borrower's
                  Total Liabilities which mature within one (1) year, including,
                  without duplication, all obligations and liabilities of
                  Borrower and SpeechWorks to Silicon, minus only for the period
                  beginning March 29, 2005 through September 30, 2005: (i) all
                  obligations and liabilities of Borrower to Philips pursuant to
                  a certain convertible subordinated debenture dated as of
                  January 30, 2003 in the amount of Twenty-Seven Million Five
                  Hundred Thousand Dollars ($27,500,000.00), and (ii) all
                  obligations and liabilities of Borrower to Advanced
                  Recognition Technologies, Inc. in connection with Borrower's
                  acquisition of Advanced Recognition Technologies, Inc. in the
                  amount of Sixteen Million Five Hundred Thousand Dollars
                  ($16,500,000.00) (collectively, Borrower's obligations and
                  liabilities to Philips and Advanced Recognition Technologies,
                  Inc. is the "Excluded Debt")."

4.    CONSENT. Notwithstanding the terms of the Loan Agreement, Bank hereby
      consents to the Excluded Debt provided that it remains unsecured.

5.    FEES. Borrower shall reimburse Bank for all legal fees and expenses
      incurred in connection with this amendment to the Existing Loan Documents.

<PAGE>

                                                                    EXHIBIT 10.4

6.    RATIFICATION OF NEGATIVE PLEDGE AGREEMENT. Borrower hereby ratifies,
      confirms and reaffirms, all and singular, the terms and conditions of the
      Negative Pledge Agreement and acknowledges, confirms and agrees that the
      Negative Pledge Agreement remains in full force and effect.

7.    CONSISTENT CHANGES. The Existing Loan Documents are hereby amended
      wherever necessary to reflect the changes described above.

8.    RATIFICATION OF LOAN DOCUMENTS. Borrower hereby ratifies, confirms, and
      reaffirms all terms and conditions of all security or other collateral
      granted to the Bank, and confirms that the indebtedness secured thereby
      includes, without limitation, the Obligations.

9.    NO DEFENSES OF BORROWER. Borrower hereby acknowledges and agrees that
      Borrower has no offsets, defenses, claims, or counterclaims against Bank
      with respect to the Obligations, or otherwise, and that if Borrower now
      has, or ever did have, any offsets, defenses, claims, or counterclaims
      against Bank, whether known or unknown, at law or in equity, all of them
      are hereby expressly WAIVED and Borrower hereby RELEASES Bank from any
      liability thereunder.

10.   CONTINUING VALIDITY. Borrower understands and agrees that in modifying the
      existing Obligations, Bank is relying upon Borrower's representations,
      warranties, and agreements, as set forth in the Existing Loan Documents.
      Except as expressly modified pursuant to this Loan Modification Agreement,
      the terms of the Existing Loan Documents remain unchanged and in full
      force and effect. Bank's agreement to modifications to the existing
      Obligations pursuant to this Loan Modification Agreement in no way shall
      obligate Bank to make any future modifications to the Obligations. Nothing
      in this Loan Modification Agreement shall constitute a satisfaction of the
      Obligations. It is the intention of Bank and Borrower to retain as liable
      parties all makers of Existing Loan Documents, unless the party is
      expressly released by Bank in writing. No maker will be released by virtue
      of this Loan Modification Agreement.

11.   JURISDICTION/VENUE. Borrower accepts for itself and in connection with its
      properties, unconditionally, the non-exclusive jurisdiction of any state
      or federal court of competent jurisdiction in the Commonwealth of
      Massachusetts in any action, suit, or proceeding of any kind against it
      which arises out of or by reason of this Loan Modification Agreement;
      provided, however, that if for any reason Bank cannot avail itself of the
      courts of the Commonwealth of Massachusetts, then venue shall lie in Santa
      Clara County, California.

12.   COUNTERSIGNATURE/EFFECTIVENESS. This Loan Modification Agreement shall
      become effective only when it shall have been executed by Borrower and
      Bank.

<PAGE>

                                                                    EXHIBIT 10.4

      This Loan Modification Agreement is executed as a sealed instrument under
the laws of the Commonwealth of Massachusetts as of the date first written
above.

                                        BORROWER:

                                        SCANSOFT, INC.

                                        By:  /s/ Jamie Arnold Jr.
                                           ________________________________
                                        Name:    Jamie Arnold Jr.
                                        Title:   CFO ScanSoft Inc.

                                        BANK:

                                        SILICON VALLEY BANK, d/b/a
                                        SILICON VALLEY EAST

                                        By:  /s/ Pamela Braren
                                           _________________________________
                                        Name:    Pamela Braren
                                        Title:   VP Silicon Valley Bank

                                        SILICON VALLEY BANK

                                        By:  /s/ Jacquelyn Le
                                           _________________________________
                                        Name:    Jacquelyn Le
                                        Title:   Operations Supervisor
                                                 Silicon Valley Bank
<PAGE>

                                                                    EXHIBIT 10.4

      The undersigned ratifies, confirms and reaffirms, all and singular, the
terms and conditions of a certain Unconditional Guaranty dated October 31, 2002
(the "Guaranty") and a certain Security Agreement dated October 31, 2002 (the
"Security Agreement") and acknowledges, confirms and agrees that the Guaranty
and the Security Agreement shall remain in full force and effect and shall in no
way be limited by the execution of this Loan Modification Agreement, or any
other documents, instruments and/or agreements executed and/or delivered in
connection herewith.

CAERE CORPORATION

By:    /s/ Jamie Arnold Jr.
     ______________________________________
Name:      Jamie Arnold Jr.
       ____________________________________
Title:     as CFO of Parent ScanSoft Inc.
        ___________________________________

      The undersigned each hereby ratifies, confirms and reaffirms, all and
singular, the terms and conditions of certain Unconditional Guarantees each
dated September 30, 2003 (collectively, the "Guaranty") and acknowledges,
confirms and agrees that the Guaranty shall remain in full force and effect and
shall in no way be limited by the execution of this Loan Modification Agreement,
or any other documents, instruments and/or agreements executed and/or delivered
in connection herewith.

SPEECHWORKS SECURITIES CORP.


By:    /s/ Jamie Arnold Jr.
     ______________________________________
Name:      Jamie Arnold Jr.
       ____________________________________
Title:     as CFO of Parent ScanSoft Inc.
        ___________________________________


SPEECHWORKS INTERNATIONAL HOLDINGS, INC.


By:    /s/ Jamie Arnold Jr.
     ______________________________________
Name:      Jamie Arnold Jr.
       ____________________________________
Title:     as CFO of Parent ScanSoft Inc.
        ___________________________________


SPEECHWORKS ACQUISITION CORP.


By:    /s/ Jamie Arnold Jr.
     ______________________________________
Name:      Jamie Arnold Jr.
       ____________________________________
Title:     as CFO of Parent ScanSoft Inc.
        ___________________________________


SPEECHWORKS INTERNATIONAL, INC.


By:    /s/ Jamie Arnold Jr.
     ______________________________________
Name:      Jamie Arnold Jr.
       ____________________________________
Title:     as CFO of Parent ScanSoft Inc.
        ___________________________________