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                                                                   EXHIBIT 10.20

 
                         FOURTH MODIFICATION AGREEMENT
                         -----------------------------



DATE:                February 28, 1999
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PARTIES:  Borrower:  MATRIX CAPITAL CORPORATION,
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                     a Colorado corporation


          Bank:      BANK ONE, ARIZONA, NA,
                     a national banking association

RECITALS:
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     A.  Bank has extended to Borrower credit ("Loan") in the principal amount
                                                ----                          
of $980,000.00 pursuant to the Loan Agreement, dated June 29, 1995 ("Loan
                                                                     ----
Agreement"), and evidenced by the Promissory Note, dated June 29, 1995 ("Note").
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The unpaid principal of the Loan as of the date hereof is $870,477.00.

     B.  The Loan is secured by, among other things, the Deed of Trust,
Assignment of Rents, Security Agreement and Fixture Filing, dated June 29, 1995
("Deed of Trust"), by Borrower, as trustor, for the benefit of Bank, as
  -------------                                                        
beneficiary, recorded on July 3, 1995, Instrument No. 95-0387291,  records of
Maricopa County, Arizona (the agreements, documents, and securing the Loan and
the Note are referred to individually and collectively as the "Security
                                                               --------
Documents").
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     C.  Bank and Borrower have executed and delivered previously the following
agreements ("Modifications") modifying the terms of the Loan, the Note, the Loan
             -------------                                                      
Agreement, and/or the Security Documents: (i) the First Modification Agreement
dated June 26, 1998, (ii) the Second Modification Agreement dated September 29,
1998, (iii) the Third Modification Agreement dated November 29, 1998.  (The
Note, the Loan Agreement, the Security Documents, any arbitration resolution,
any environmental certification and indemnity agreement, and all other
agreements, documents, and instruments evidencing, securing, or otherwise
relating to the Loan, as modified in the Modifications, are sometimes referred
to individually and collectively as the "Loan Documents".  Hereinafter, "Note",
                                         --------------                  ----  
"Loan Agreement", "Deed of Trust", and "Security Documents" shall mean such
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documents as modified in the Modifications.)
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     D.  Borrower has requested that Bank modify the Loan and the Loan Documents
as provided herein.  Bank is willing to so modify the Loan and the Loan
Documents, subject to the terms and conditions herein.

AGREEMENT:
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For good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, Borrower and Bank agree as follows:

1.  ACCURACY OF RECITALS.
    -------------------- 

Borrower acknowledges the accuracy of the Recitals.

2.  MODIFICATION OF LOAN DOCUMENTS.
    ------------------------------ 

    2.1  The Loan Documents are modified as follows:

    2.1.1  The maturity date of the Loan and the Note is changed from February
28, 1999, to June 30, 1999.  On the maturity date Borrower shall pay to Bank the
unpaid principal, accrued and unpaid interest, and all other amounts payable by
Borrower under the Loan Documents as modified herein.

    2.2  Each of the Loan Documents is modified to provide that it shall be a
default or an event of default thereunder if Borrower shall fail to comply with
any of the covenants of Borrower herein or if any representation or warranty by
Borrower herein or by any guarantor in any related Consent and Agreement of
Guarantor(s) is materially incomplete, incorrect, or misleading as of the date
hereof.

    2.3  Each reference in the Loan Documents to any of the Loan Documents
shall be a reference to such document as modified herein.

3.  RATIFICATION OF LOAN DOCUMENTS AND COLLATERAL.
    --------------------------------------------- 

The Loan Documents are ratified and affirmed by Borrower and shall remain in
full force and effect as modified herein.  Any property or rights to or
interests in property granted as security in the Loan Documents shall remain as
security for the Loan and the obligations of Borrower in the Loan Documents.

4.  BORROWER REPRESENTATIONS AND WARRANTIES.
    --------------------------------------- 

Borrower represents and warrants to Bank:

    4.1  No default or event of default under any of the Loan Documents as
modified herein, nor any event, that, with the giving of notice or the passage
of 
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time or both, would be a default or an event of default under the Loan
Documents as modified herein has occurred and is continuing.

    4.2  There has been no material adverse change in the financial condition
of Borrower or any other person whose financial statement has been delivered to
Bank in connection with the Loan from the most recent financial statement
received by Bank.

    4.3  Each and all representations and warranties of Borrower in the Loan
Documents are accurate on the date hereof.

    4.4  Borrower has no claims, counterclaims, defenses, or set-offs with
respect to the Loan or the Loan Documents as modified herein.

    4.5  The Loan Documents as modified herein are the legal, valid, and
binding obligation of Borrower, enforceable against Borrower in accordance with
their terms.

    4.6  Borrower is validly existing under the laws of the State of its
formation or organization and has the requisite power and authority to execute
and deliver this Agreement and to perform the Loan Documents as modified herein.
The execution and delivery of this Agreement and the performance of the Loan
Documents as modified herein have been duly authorized by all requisite action
by or on behalf of Borrower.  This Agreement has been duly executed and
delivered on behalf of Borrower.

5.  BORROWER COVENANTS.
    ------------------ 

Borrower covenants with Bank:

    5.1  Borrower shall execute, deliver, and provide to Bank such additional
agreements, documents, and instruments as reasonably required by Bank to
effectuate the intent of this Agreement.

    5.2  Borrower fully, finally, and forever releases and discharges Bank and
its successors, assigns, directors, officers, employees, agents, and
representatives from any and all actions, causes of action, claims, debts,
demands, liabilities, obligations, and suits, of whatever kind or nature, in law
or equity of Borrower, whether now known or unknown to Borrower, (i) in respect
of the Loan, the Loan Documents, or the actions or omissions of Bank in respect
of the Loan or the Loan Documents and (ii) arising from events occurring prior
to the date of this Agreement.

    5.3   Contemporaneously with the execution and delivery of this Agreement,
Borrower has paid to Bank:
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    5.3.1  All accrued and unpaid interest under the Note and all amounts,
other than interest and principal, due and payable by Borrower under the Loan
Documents as of the date hereof.

    5.3.2  A modification fee of $1,000.00.

6.  EXECUTION AND DELIVERY OF AGREEMENT BY BANK.
    ------------------------------------------- 

Bank shall not be bound by this Agreement until (i) Bank has executed and
delivered this Agreement, (ii) Borrower has performed all of the obligations of
Borrower under this Agreement to be performed contemporaneously with the
execution and delivery of this Agreement, (iii) each guarantor(s) of the Loan,
if any, has executed and delivered to Bank a Consent and Agreement of
Guarantor(s), and (iv) if required by Bank, Borrower and any guarantor(s) have
executed and delivered to Bank an arbitration resolution, an environmental
questionnaire, and an environmental certification and indemnity agreement.

7.   ENTIRE AGREEMENT, CHANGE, DISCHARGE, TERMINATION, OR WAIVER.
     ----------------------------------------------------------- 

The Loan Documents as modified herein contain the entire understanding and
agreement of Borrower and Bank in respect of the Loan and supersede all prior
representations, warranties, agreements, arrangements, and understandings.  No
provision of the Loan Documents as modified herein may be changed, discharged,
supplemented, terminated, or waived except in a writing signed by Bank and
Borrower.

8.  BINDING EFFECT.
    -------------- 
 
The Loan Documents as modified herein shall be binding upon, and inure to the
benefit of, Borrower and Bank and their respective successors and assigns.

9.  CHOICE OF LAW.
    ------------- 

This Agreement shall be governed by and construed in accordance with the laws of
the State of Arizona, without giving effect to conflicts of law principles.

10.  COUNTERPART EXECUTION.
     --------------------- 

This Agreement may be executed in one or more counterparts, each of which shall
be deemed an original and all of which together shall constitute one and the
same document.  Signature pages may be detached from the counterparts and
attached to a single copy of this Agreement to physically form one document.

DATED March 19, 1999, but effective as of the date first above stated.
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                                MATRIX CAPITAL CORPORATION, a 
                                Colorado corporation

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


                                BANK ONE, ARIZONA, NA, a national banking 
                                association


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