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                                                                    Exhibit 10.4
 
                FOURTH AMENDMENT TO REVOLVING CREDIT AGREEMENT
                ----------------------------------------------

     This Fourth Amendment (this "Fourth Amendment") to Revolving Credit
Agreement is made and entered into as of the 31st day of October, 1997, by and
among BOSTON PROPERTIES LIMITED PARTNERSHIP ("BPLP") and the Wholly-owned
Subsidiaries which are listed on Schedule 1 to a certain Revolving Credit
Agreement (as amended, the "Credit Agreement") dated as of June 23, 1997, each
having its principal place at 8 Arlington Street, Boston, Massachusetts 02116,
BANKBOSTON, N.A. ("BankBoston"), a national banking association, having its
principal place of business at 100 Federal Street, Boston, Massachusetts 02110
and certain other lending institutions (collectively with BankBoston, the
"Banks") and BANKBOSTON, N.A., as agent for itself and each other Bank.

     NOW, THEREFORE, in consideration of One Dollar ($1.00) and other good and
valuable consideration by each of the parties hereto, the receipt and
sufficiency of which are hereby acknowledged, it is agreed as follows:

     1.       Capitalized terms used but not defined herein shall have the
          respective meanings assigned to such terms in the Credit Agreement.

     2.       The definition of "Applicable Margin" is amended, solely for the
          period commencing on the 875 Third Avenue Closing Date and ending on
          April 30, 1998, by adding the following to the end of the existing
          definition of "Applicable Margin":

                   "Notwithstanding the foregoing, the Applicable Margin shall
             be equal to the percentage determined for the following periods in
             accordance with the following:

                   (i) For the period from the 875 Third Avenue Closing Date
             through January 31, 1998, the Applicable Margin will equal 1.25%.

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                   (ii) For the period from February 1, 1998 through April 30,
             1998 the Applicable Margin will equal 1.40%."

     3.        Section 10.1 of the Credit Agreement is amended, solely for the
          period commencing on the 875 Third Avenue Closing Date and ending on
          April 30, 1998, by deleting the reference to "55%" and by inserting in
          place thereof the following new reference: "65%".

     4.        Section 10.2 of the Credit Agreement is amended, solely for the
          period commencing on the 875 Third Avenue Closing Date and ending on
          April 30, 1998, by deleting the reference to "50%" contained in clause
          (i) thereof (as amended in Paragraph 4 of the Third Amendment to
          Revolving Credit Agreement dated September 11, 1997) and by inserting
          in place thereof the following new reference: "55%."

     5.        The Borrower hereby represents and warrants to you as follows:

          (a) Representations in Credit Agreement.  Each of the representations 
              -----------------------------------   
          and warranties made by or on behalf of the Borrower, the Guarantor or
          any of their respective Subsidiaries contained in the Credit
          Agreement, as amended through this Fourth Amendment, or any of the
          other Loan Documents, was true when made and is true on and as of the
          date hereof with the same full force and effect as if each of such
          representations and warranties had been made on the date hereof and in
          this Fourth Amendment, except (i) to the extent of changes resulting
          from transactions contemplated or not prohibited by the Credit
          Agreement or the other Loan Documents (including, without limitation,
          the fact that a Real Estate Asset may cease to be a Borrowing Base
          Property pursuant to the terms of the Credit Agreement) and changes
          occurring in the ordinary course of business, (ii) to the extent that
          such representations and warranties relate expressly to an earlier
          date and (iii) to the extent otherwise represented by the Borrower
          with respect to the representation set forth in Section 7.10.

          (b) No Events of Default.  No Default or Event of Default exists on 
              -------------------- 
          the date hereof.

          (c) Binding Effect of Documents.  This Fourth Amendment has been duly 
              --------------------------- 
          executed and delivered to you by the Borrower and is in full force and

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          effect as of the date hereof, and the agreements and obligations of
          the Borrower contained herein constitute legal, valid and binding
          obligations of the Borrower enforceable against the Borrower in
          accordance with their respective terms.

     6.        Provisions of General Application.
               --------------------------------- 

          (a) No Other Changes.  Except as otherwise expressly provided by this 
              ----------------  
          Fourth Amendment, all of the terms, conditions and provisions of the
          Credit Agreement and each of the other Loan Documents remain
          unaltered. The Credit Agreement and this Fourth Amendment shall be
          read and construed as one agreement.

          (b) Governing Law.  This Fourth Amendment is intended to take effect 
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          as a sealed instrument and shall be deemed to be a contract under the
          laws of the Commonwealth of Massachusetts. This Fourth Amendment and
          the rights and obligations of each of the parties hereto shall be
          governed by and interpreted and determined in accordance with the laws
          of the Commonwealth of Massachusetts.

          (c) Binding Effect; Assignment.  This Fourth Amendment shall be 
              --------------------------
          binding upon and inure to the benefit of each of the parties hereto
          and their respective successors in title and assigns.

          (d) Counterparts.  This Fourth Amendment may be executed in any number
              ------------
          of counterparts, but all such counterparts shall together constitute
          but one and the same agreement. In making proof of this Fourth
          Amendment, it shall not be necessary to produce or account for more
          than one counterpart thereof signed by each of the parties hereto.

          (e) Conflict with Other Agreements.  If any of the terms of this 
              ------------------------------ 
          Fourth Amendment shall conflict in any respect with any of the terms
          of any of the Credit Agreement or any other Loan Document, the terms
          of this Fourth Amendment shall be controlling.

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     WITNESS the execution hereof, under seal, as of the day and year first
written above.

[Signature pages omitted.  Executed by the following parties:]

                                       BANKBOSTON, N.A.,
                                       individually and as Lead Agent

                                       CITIZENS BANK

                                       THE SAKURA BANK, LIMITED

                                       THE BANK OF NOVA SCOTIA

                                       FLEET NATIONAL BANK, individually
                                       and as Documentation Agent

                                       THE CHASE MANHATTAN BANK, individually
                                       and as Syndication Agent

                                       SUMMIT BANK


                                       PNC BANK NATIONAL ASSOCIATION, 
                                       individually and as a Co-Agent

                                       DRESDNER BANK AG NEW YORK BRANCH AND 
                                       GRAND CAYMAN BRANCH, individually and 
                                       as a Co-Agent

                                       THE BANK OF NEW YORK
                                       CRESTAR BANK

                                       USTRUST

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                                       KEY BANK NATIONAL ASSOCIATION
                                                                    
                                       THE ATLANTIC MONTHLY TRUST   
                                                                    
                                       MBZ-LEX TRUST                
                                                                    
                                       ZEE EM TRUST II              
                                                                    
                                       WP TRUST                     
                                                                    
                                       TRACER LANE TRUST            
                                                                    
                                       HAYDEN OFFICE TRUST          
                                                                    
                                       ELANDZEE TRUST               
                                                                    
                                       ZEE BEE TRUST II             
                                                                    
                                       ONE CAMBRIDGE CENTER TRUST   
                                                                    
                                       THREE CAMBRIDGE CENTER TRUST 
                                                                    
                                       ELEVEN CAMBRIDGE CENTER TRUST
                                                                    
                                       FOURTEEN CAMBRIDGE CENTER TRUST 
                                                                       
                                       SCHOOL STREET ASSOCIATES LIMITED 
                                       PARTNERSHIP               
                                       
                                       DEMOCRACY ASSOCIATES LIMITED 
                                       PARTNERSHIP                   

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                                       MARYLAND 50 BUILDING I ASSOCIATES 
                                       LIMITED PARTNERSHIP      
                                       
                                       MARYLAND 50 BUILDING II ASSOCIATES 
                                       LIMITED PARTNERSHIP     
                                       
                                       MARYLAND 50 BUILDING III ASSOCIATES 
                                       LIMITED PARTNERSHIP    
                                       
                                       DOWNTOWN BOSTON PROPERTIES TRUST 
                                                                        
                                       TWO CAMBRIDGE CENTER TRUST       
                                                                        
                                       BOSTON PROPERTIES LIMITED PARTNERSHIP 
                                                                             
                                       BOSTON PROPERTIES, INC.,              
                                       as Guarantor                          

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