<PAGE>   1
                                                                 Exhibit 10.12d


                                 AMENDMENT NO. 4
                                       TO
                                CREDIT AGREEMENT

         AMENDMENT NO. 4 TO CREDIT AGREEMENT (the "AMENDMENT"), dated as of
April 14, 1999, among American Architectural Products Corporation, a Delaware
corporation, Eagle & Taylor Company, a Delaware corporation, Forte, Inc., an
Ohio corporation, Western Insulated Glass, Co., an Arizona corporation,
Thermetic Glass, Inc., a Delaware corporation, Binnings Building Products, Inc.,
a Delaware corporation, Danvid Window Company, a Delaware corporation. Modern
Window Acquisition Corporation, a Delaware corporation, American Glassmith
Acquisition Corporation, a Delaware corporation, VinylSource, Inc., a Delaware
corporation, Weather-Seal Acquisition Corporation, a Delaware corporation, Eagle
Window & Door Center, Inc., a Delaware corporation, Denver Window Acquisition
Corporation, a Delaware corporation, AAPC One Acquisition Corporation, a
Delaware corporation, and AAPC Two Acquisition Corporation, a Delaware
corporation (the "BORROWERS"), the institutions party hereto (the "LENDERS"),
and BankBoston, N.A., in its capacity as contractual representative for itself
and the other Lenders (the "AGENT") under that certain Credit Agreement, dated
as of June 9, 1998, as amended, by and among the Borrowers, the Lenders and the
Agent (the "CREDIT AGREEMENT"). Defined terms used herein and not otherwise
defined herein shall have the meaning given to them in the Credit Agreement.

         WHEREAS, the Borrowers, the Lenders and the Agent have entered into the
 Credit Agreement; and

         WHEREAS, the Borrowers, the Lenders and the Agent have agreed to amend
 the Credit Agreement on the terms and conditions set forth herein.

         NOW, THEREFORE, in consideration of the premises set forth above, and
 for other good and valuable consideration, the receipt and sufficiency of which
 are hereby acknowledged, the Borrowers, the Lenders and the Agent agree as
 follows:

         1. Amendment to the Credit Agreement. Effective as of the date first
 above written and subject to the execution of this Amendment by the parties
 hereto and the satisfaction of the conditions precedent set forth in Section 2
 below, the Credit Agreement shall be and hereby is amended as follows:

         1.1. Section 7.3(S) is hereby amended and restated as follows:

         "(S) Limitation on Revolving Credit Availability. The Borrowers shall
         not permit the Revolving Credit Availability to be less than Ten
         Million Dollars ($10,000,000.00) between April 30, 1999 and June 1,
         1999, and, after June I, 1999, at any time between April 15 and June I
         or between October 15 and December 1 of any year thereafter.


<PAGE>   2

         2. Conditions Precedent. This Amendment shall become effective as of
the date above written, if, and only if, the Agent has received duly executed
originals of this Amendment from the Borrowers, the Required Lenders and the
Agent.

         3. Representations and Warranties of the Borrowers. The Borrowers
hereby represent and warrant as follows:

         (a) This Amendment and the Credit Agreement, as amended hereby,
constitute legal, valid and binding obligations of the Borrowers and are
enforceable against the Borrowers in accordance with their terms.

         (b) Upon the effectiveness of this Amendment, the Borrowers hereby
reaffirm all representations and warranties made in the Credit Agreement, and to
the extent the same are not amended hereby, agree that all such representations
and warranties shall be deemed to have been remade as of the date of delivery of
this Amendment, unless and to the extent that any such representation and
warranty is stated to relate solely to an earlier date, in which case such
representation and warranty shall be true and correct as of such earlier date.

         4. Reference to and Effect on the Credit Agreement.

         (a) Upon the effectiveness of Section I hereof, on and after the date
hereof, each reference in the Credit Agreement to "this Credit Agreement,"
"hereunder," "hereof," "herein" or words of like import shall mean and be a
reference to the Credit Agreement as amended hereby.

         (b) The Credit Agreement, as amended hereby, and all other documents,
instruments and agreements executed and/or delivered in connection therewith,
shall remain in full force and effect, and are hereby ratified and confirmed.

         (c) Except as expressly provided herein, the execution, delivery and
effectiveness of this Amendment shall not operate as a waiver of any right,
power or remedy of the Agent or the Lenders, nor constitute a waiver of any
provision of the Credit Agreement or any other documents, instruments and
agreements executed and/or delivered in connection therewith.

         5. Governing Law. This Amendment shall be governed by and construed in
accordance with the internal laws (as opposed to the conflict of law provisions)
of the State of Illinois.

         6. Headings. Section headings in this Amendment are included herein for
convenience of reference only and shall not constitute a part of this Amendment
for any other purpose.

         7. Counterparts. This Amendment may be executed by one or more of the
parties to the Amendment on any number of separate counterparts and all of said
counterparts taken together shall be deemed to constitute one and the same
instrument.


                                    -2-
<PAGE>   3



         IN WITNESS WHEREOF, this Amendment has been duly executed and delivered
on the date first above written.


AMERICAN ARCHITECTURAL                      BANKBOSTON, N.A., individually
PRODUCTS CORPORATION                                 and as Agent

EAGLE AND TAYLOR COMPANY                    By:     /s/ W.J. Sherald  
                                                --------------------------
                                                 Title: Vice President
FORTE, INC.

WESTERN INSULATED GLASS, CO.

THERMETIC GLASS, INC.

BINNINGS BUILDING PRODUCTS, INC.

DANVID WINDOW COMPANY

MODERN WINDOW ACQUISITION
CORPORATION

AMERICAN GLASSMITH ACQUISITION
CORPORATION

VINYLSOURCE, INC.

WEATHER-SEAL ACQUISITION
CORPORATION

EAGLE WINDOW & DOOR CENTER, INC.

DENVER WINDOW ACQUISITION
CORPORATION

AAPC ONE ACQUISITION CORPORATION

AAPC TWO ACQUISITION CORPORATION


By:         /s/ Richard L. Kovach              
    --------------------------------------------     
         (on behalf of the above-listed parties)
         Name: Richard L. Kovach
         Title: CFO



                                    -3-