Exhibit 4

          THIRD AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT

Harris Trust and Savings Bank                  Union Bank of California, N.A.
Chicago, Illinois                              San Francisco, California

LaSalle National Bank                          Bank One, Arizona, N.A.
Chicago, Illinois                              Phoenix, Arizona

First Union National Bank                      BankBoston, N.A.
Philadelphia, Pennsylvania                     Stamford, Connecticut


Ladies and Gentlemen:

         Reference is hereby made to that certain  Amendment and  Restatement of
Credit  Agreement  dated as of  December  22,  1998,  as  amended  (such  Credit
Agreement  as  heretofore  amended  being  referred  to  herein  as the  "Credit
Agreement") among the undersigned,  EMCOR Group,  Inc., a Delaware  corporation,
Comstock  Canada  Ltd.,  A Canadian  corporation,  and Drake & Scull  Group Ltd.
(formerly named Drake & Scull  Engineering  Ltd.), a United Kingdom  corporation
(collectively,  the  "Borrowers" and  individually,  the  "Borrower"),  you (the
"Lenders")  and Harris  Trust and Savings  Bank,  as agent for the Lenders  (the
"Agent").  All defined  terms used herein  shall have the same meaning as in the
Credit Agreement unless otherwise define herein.

         The  Borrowers,  the Agent and the Lenders wish to modify certain terms
and  conditions of the Credit  Agreement,  all on the terms and  conditions  set
forth in this Amendment.

SECTION 1.                 AMENDMENTS TO CREDIT AGREEMENT

         Upon  satisfaction  of all of the  conditions  precedent  set  forth in
Section 3 hereof, the Credit Agreement shall be amended as follows:

1.1 The definition of "Borrowers" set forth in Section 9 of the Credit Agreement
shall be amended in its entirety and as so amended  shall be restated to read as
follows:

                  "Borrowers"  means (a) the U.S.  Borrowers,  (b) the  Canadian
         Borrowers and (c) the U.K. Borrowers,  with (i) the term "Borrowers" to
         mean the Borrowers, collectively, and, also each individually, and (ii)
         all   promises   and   covenants   (including   promises  to  pay)  and
         representations and warranties of and by the Borrowers made in the Loan
         Documents or any instruments or documents delivered pursuant thereto to
         be  and   constitute  the  joint  and  several   promises,   covenants,
         representations  and  warranties  of  and  by  each  and  all  of  such
         corporations,  except to the extent explicitly otherwise provided.  The
         term  "Borrower"  appearing  in such  singular  form  shall be deemed a
         reference to any of the Borrowers unless the context in which such term
         is used shall otherwise require.

          1.2 The  definition of "Borrowing  Base" set forth in Section 9 of the
Credit  Agreement  shall be  amended by adding the  following  sentence  thereto
immediately at the end thereof:

                  Anything  contained in this Agreement  notwithstanding  (i) in
         computing   compliance  by  U.S.  Borrowers  with  the  Borrowing  Base
         requirements set forth in this Agreement,  Eligible Accounts Receivable
         shall only include those Eligible Accounts  Receivable  attributable to
         the U.S.  Borrowers  and the U.S.  Subsidiaries  (exclusive of Eligible
         Accounts  Receivable of the Company used to support Credit  Utilization
         of the Canadian  Borrowers and/or the U.K. Borrowers pursuant to clause
         (ii) of this sentence) and (ii) for purposes of computing compliance by
         the Canadian  Borrowers and the U.K.  Borrowers with the Borrowing Base
         requirements  set  forth  herein  Eligible  Accounts  Receivable  shall
         include  only  those  Eligible  Accounts  Receivable   attributable  to
         Restricted  Subsidiaries  which are not U.S.  Subsidiaries and Eligible
         Accounts  Receivable  of the Company,  to the extent that such Eligible
         Accounts  Receivable  of the  Company  were not used to support  Credit
         Utilizations by the U.S. Borrowers.  Deductions to be made in computing
         the Borrowing  Base in respect of amounts  recorded for costs in excess
         of billings  representing certain disputed items shall be taken against
         the allocated to the Eligible  Accounts  Receivable owing to the entity
         which has recorded such costs.

         1.3  Section  9 of the  Credit  Agreement  shall be  amended  by adding
thereto the following new definitions in the appropriate alphabetical locations:

                  "Canadian  Borrower"  means and includes  Comstock  Canada and
         such other Restricted  Subsidiaries organized under the Federal laws of
         Canada or the laws of a Province  of Canada as may from time to time be
         designated  as such in writing by the Company  and  approved as such in
         writing by all lenders (but subject to such  conditions and limitations
         as either the Company or the Lenders may impose).

                  "Canadian Subsidiaries" means and includes Comstock Canada and
         such other  Subsidiaries  organized under the Federal laws of Canada or
         the laws of a Province of Canada.

                  "U.K  Borrowers"  means  and  includes  Drake & Scull and such
         other  Restricted  Subsidiaries  organized under the laws of the United
         Kingdom  as may from time to time be  designated  as such in writing by
         the Company and approved as such in writing by all lenders (but subject
         to such conditions and limitations as either the Company or the Lenders
         may impose).

                  "U.K.  Subsidiaries"  means  Drake  & Scull  and  such  other
         Subsidiaries organized under the laws of the United Kingdom.

                  "U.S.  Borrowers"  mean the Company and such other  Restricted
         Subsidiaries  organized  under the laws of the United States of America
         as may  from  time to  time be  designated  as such in  writing  by the
         Company and  approved as such in writing by all Lenders (but subject to
         such  conditions  and  limitations as either the Company or Lenders may
         impose).

                  "U.S.  Subsidiaries"  means the  Subsidiaries  of the  Company
         organized  under the laws of the  United  States of America as may from
         time to time be  designated  as such  in  writing  by the  Company  and
         approved  as  such in  writing  by all  Lenders  (but  subject  to such
         conditions  and  limitations  as either  the  Company  or  Lenders  may
         impose).

         1.4      Section 4.1 of the Credit Agreement shall be amended by adding
thereto the following language  immediately at the end thereof:

               "Notwithstanding  anything to the contrary contained herein or in
          any  other  Loan  Document,  the  Collateral  (other  than  Collateral
          constituting  capital  stock  of the  Guarantors)  owned  by the  U.K.
          Subsidiaries  and the Canadian  Subsidiaries  shall secure  solely the
          indebtedness, liabilities and obligations of the U.K. Subsidiaries and
          the Canadian Subsidiaries hereunder and under the other Loan Documents
          and not the  indebtedness,  liabilities  and  obligations  of the U.S.
          Borrowers and the U.S. Subsidiaries hereunder and under the other Loan
          Documents. The portion of the capital stock of each Guarantor which is
          a U. K. Subsidiary or a Canadian Subsidiary constituting Collateral in
          excess of 65% of the total  issued and  outstanding  capital  stock of
          such Subsidiary  (herein,  the "Excess Stock Collateral") shall secure
          only the  indebtedness  liabilities  and  obligations  of the Canadian
          Subsidiaries  and/or U.K.  Subsidiaries  hereunder and under the other
          Loan Documents.  In no event shall the Excess Stock Collateral  secure
          the indebtedness, liabilities and obligations of the U.S. Borrowers or
          the U.S. Subsidiaries hereunder or under the other Loan documents.  It
          is  understood  that,  subject to compliance  with the Borrowing  Base
          restrictions  set forth above, the Company may borrow to fund loans to
          Restricted Subsidiaries permitted by Section 7.12 hererof."

SECTION 2.  RELEASE OF GUARANTEES

         Notwithstanding anything contained in the Credit Agreement of the other
Loan Documents to the contrary,  the Guarantees executed by the Guarantors which
are U.K.  Subsidiaries  or Canadian  Subsidiaries  shall in no event be deemed a
guaranty of the indebtedness, liabilities and obligations of the U. S. Borrowers
or the U.S.  Subsidiaries under the Credit Agreement or the other Loan Documents
and such Guarantees shall be deemed released as to the indebtedness, liabilities
and obligations of the U.S. Borrowers of the U.S.  Subsidiaries under the Credit
Agreement and the other Loan Documents, but not otherwise.

SECTION 3.        WAIVER

         Section 4.1 of the Credit Agreement to the contrary  notwithstanding no
lien need be granted on the stock of the  Canadian  Subsidiaries,  Drake & Scull
engineering Ltd. (formerly named Drake & Scull Engineering (north) Ltd. or Drake
&  Scull  Ltd.   (formerly  named  Drake  &  Scull   Engineering   (South)  Ltd.
(collectively,  "New UK  Companies")  until  July 31,  2000,  (ii) the  Canadian
Subsidiaries need not grant a lien on their assets unless and until the Canadian
Borrowers  desire to include  the  assets of the  Canadian  Subsidiaries  in the
Borrowing  Base and  (iii) the New UK  Companies  need not grant a lien on their
assets unless and until the UK Borrowers desire to include the assets of the New
UK Companies in the Borrowing Base.

SECTION 4.        CONDITIONS PRECEDENT

         The  effectiveness  of this Amendment is subject to the satisfaction of
all of the following conditions precedent:

         4.1 The  Borrowers,  the  Agent and the  Required  Lenders  shall  have
executed this Amendment (such execution may be in several  counterparts  and the
several parties hereto may execute on separate counterparts).

         4.2 A  Guarantor's  Consent for the  benefit of the Lenders  shall have
been executed and delivered to the Agent, the form of which is attached hereto.

         4.3 The Borrowers  shall be  in full  compliance  with all of the terms
and  conditions  of the  Loan  Documents  and  no  Default  or Event of  Default
shall have  occurred and be  continuing  thereunder or shall result after giving
effect to this Amendment.

         4.4 Legal  matters  incident  to the  execution  and  delivery  of this
Amendment shall be satisfactory to each of the Lenders and their legal counsel.

SECTION 5.        MISCELLANEOUS

         5.1 Each of the Borrowers has heretofore  executed and delivered to the
Agent that certain Amended and Restated Security  Agreement dated as of December
22,  1998 (the  "Security  Agreement")  and each  Borrower  hereby  agrees  that
notwithstanding the execution and delivery hereof, such Security Agreement shall
be and remain in full force and effect and that any rights and  remedies  of the
Agent  thereunder,  obligations  of the  Borrowers  thereunder  and any liens or
security  interests  created or provided for  thereunder  shall be and remain in
full force and effect, shall not be affected, impaired or discharged thereby and
shall secure all of its  indebtedness,  obligations and liabilities to the Agent
and the Lenders under the Credit  Agreement as amended  hereby.  Nothing  herein
contained  shall in any manner  affect or impair the  priority  of the liens and
security  interests created and provided for by the Security Agreement as to the
indebtedness which would be secured thereby prior to giving effect hereto.

         5.2 Reference to this specific  Amendment need not be made in any note,
document,  letter,  certificate,  any security  agreement,  or any communication
issued  or made  pursuant  to or  with  respect  to the  Credit  Agreement,  any
reference  to the  Credit  Agreement  being  sufficient  to refer to the  Credit
Agreement as amended hereby.

         5.3 This Amendment may be executed in any number of  counterparts,  and
by the different parties on different counterparts,  all of which taken together
shall  constitute  one and the same  agreement.  Any of the  parties  hereby may
execute  this  agreement  by  signing  any  such  counterpart  and  each of such
counterparts shall for all purposes be deemed to be an original.  This agreement
shall be governed by the internal laws of the State of Illinois.

     5.4 Each of the  Borrowers  hereby  agree to pay all  reasonable  costs and
expenses, including without limitation attorneys fees, incurred by the Agent and
each of the Lenders in connection with the preparation,  negotiation,  execution
and delivery of the Amendment and the other documents contemplated hereby.


                                                     (Signature pages to follow)





         Upon  acceptance  hereof  by the Agent and the  Lenders  in the  manner
hereinafter  set forth,  this Amendment  shall be a contract  between us for the
purposes hereinabove set forth.

         Dated as of _____________, 2000


                                          EMCOR Group, Inc.

                                          By ___________________________
                                             Its __________________________


                                          COMSTOCK CANADA LTD.

                                          By ___________________________
                                             Its __________________________


                                          DRAKE & SCULL GROUP LTD.

                                          By ___________________________
                                             Its __________________________






         Accepted and agreed to as of the day and year last above written.



                                        HARRIS TRUST AND SAVINGS BANK
                                             individually and as Agent


                                        By ________________________________
                                                Its Vice President


                                        LASALLE NATIONAL BANK
                                        By ________________________________
                                           Its _______________________________


                                        FIRST UNION NATIONAL BANK

                                        By _______________________________
                                           Its ______________________________


                                        UNION BANK OF CALIFORNIA, N.A.

                                        By _______________________________
                                           Its ______________________________


                                        BANK  ONE, ARIZONA, N.A.

                                        By _______________________________
                                           Its ______________________________


                                        BANKBOSTON, N.A.

                                        By _______________________________
                                           Its ______________________________