<PAGE>   1
                                                                    EXHIBIT 99.1

                       FOURTH AMENDMENT TO LOAN DOCUMENTS

         THIS FOURTH AMENDMENT TO LOAN DOCUMENTS (this "Agreement") is made as
of the 28th day of April, 1998, by and between REPUBLIC GROUP INCORPORATED, a
Delaware corporation formerly known as Republic Gypsum Company ("Parent"),
REPUBLIC PAPERBOARD COMPANY, a Kansas corporation ("KS. Subsidiary"), REPUBLIC
PAPERBOARD COMPANY OF WEST VIRGINIA, a West Virginia Corporation ("W. VA.
Subsidiary"), REPUBLIC GYPSUM COMPANY, an Oklahoma corporation ("OK
Subsidiary"), (all of the foregoing parties are sometimes collectively referred
to as the "Borrowing Group") and NATIONSBANK, N.A., successor by merger to
BOATMEN'S FIRST NATIONAL BANK OF KANSAS CITY, a national banking association
(the "Bank");

                                    RECITALS

         A.      Bank has extended credit to Parent ("Revolving Loans")
evidenced by a Revolving Credit Promissory Note in the principal amount of
Seven Million Dollars ($7,000,000.00) dated June 30, 1995 (the "Revolving
Note").

         B.      KS. Subsidiary and W. VA. Subsidiary executed and delivered to
the Bank the Revolving Loan Guaranty Agreement dated June 30, 1995 wherein KS.
Subsidiary and W. VA. Subsidiary agreed to unconditionally guarantee to the
Bank repayment of the Revolving Loans.

         C.      Bank has made a Loan to Parent and W. VA. Subsidiary ("Term
Loan") evidenced by a Term Loan Promissory Note in the principal amount of
Twenty-Eight Million Dollars ($28,000,000.00) dated June 30, 1995 (the "Term
Note").

         D.      KS. Subsidiary executed and delivered to the Bank the Term
Loan Guaranty dated June 30, 1995 wherein KS. Subsidiary agreed to
unconditionally guarantee to the Bank the payment of certain obligations of
Parent and W. VA.  Subsidiary, including without limitation, repayment of the
Term Loan.

         E.      As security for the Term Loan, W. VA. Subsidiary granted to
the Bank liens and security interests in certain collateral as set forth in
that certain Security Agreement ("Security Agreement") dated June 30, 1995 and
that certain Deed of Trust dated June 30, 1995 and recorded June 30, 1995 with
the Clerk of the County Commission of Jefferson County, West Virginia in Book
808, at Page 493, as Document No. 4304 (the "Deed of Trust") which encumbers
certain property located in Jefferson County, West Virginia.

         F.      The credit relationship between the Bank and the Borrowing
Group is controlled and governed by the terms of the Revolving and Term Credit
Agreement dated June 30, 1995, the First Amendment To Loan Documents ("First
Amendment") dated December 1, 1995, the Second Amendment To Loan Documents
("Second Amendment") dated January 23, 1996 and the Third Amendment To Loan
Documents dated September 5, 1996 (collectively the "Credit Agreement").






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<PAGE>   2
         G.      Pursuant to the First Amendment, OK. Subsidiary agreed to
unconditionally guarantee the payment of the Term Loan and the Revolving Loans
and to become a party to the Term Loan Guaranty and the Revolving Loan
Guaranty.

         H.      The Borrowing Group and the Bank enter into this Agreement for
the purpose of (i) amending the maturity of the Revolving Note, (ii) increasing
the principal amount available for borrowing under the Revolving Note and (iii)
to amend the definition of "Applicable Margin" as contained in the Revolving
Note.

         I.      The Bank requires that the Loan Documents be expressly
ratified and confirmed by Borrowing Group.

         NOW, THEREFORE, for good and valuable consideration, receipt of which
is hereby acknowledged, and the mutual agreements contained herein, the Bank
and Borrowing Group agree as follows:

         1.      No Events of Default.  The Borrowing Group represents and
warrants that all of the representations, warranties and covenants in the
Credit Agreement remain true and correct and that no Event of Default under the
Credit Agreement or any Loan Document currently exists.

         2.      Amendments to Credit Agreement.  The Credit Agreement is
amended as follows:

         (a)     Subparagraph 2.1 is deleted and the following provision is
inserted in lieu thereof:

                 2.1      Agreement To Lend.  The Bank agrees, on the terms and
         subject to the conditions set forth in this Agreement, to make loans
         (each a "Revolving Loan") to RGC ("Revolving Loan Borrower"), from
         time to time during the period beginning on the Closing Date and
         ending on June 30, 1999 (the "Revolving Credit Maturity Date") or the
         date of termination of the commitments hereunder pursuant to Article 9
         upon the occurrence of an Event of Default, in such amounts as
         Borrower shall request as provided in Section 4.1 hereof; provided,
         however, that the Bank shall have no obligation to make a requested
         Revolving Loan if, after the making of such Revolving Loan, the
         aggregate unpaid principal balance of all Revolving Loans made by the
         Bank to Revolving Loan Borrower hereunder would exceed the Revolving
         Credit Commitment or if a Default has occurred and is continuing.
         Notwithstanding the foregoing, Revolving Loan Borrower and the Bank
         agree that on the first anniversary of the Closing Date, and on each
         anniversary of the Closing Date thereafter, the Revolving Credit
         Maturity Date may be extended for an additional one-year period as
         Revolving Loan Borrower and the Bank may mutually agree, so that at
         each such anniversary on which the





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         maturity of the Revolving Loan is extended, the Revolving Credit
         Maturity Date shall be the date that is two years after such
         anniversary.

         (b)     The reference to "Seven Million Dollars ($7,000,000.00)" in
the first recital on the first page of the Credit Agreement is deleted and
"Fifty Million Dollars ($50,000,000.00)" is inserted in lieu thereof.

         (c)     The references to "Barry Sullivan" and "(Telecopy No. (816)
691-7426)" in subparagraph 10.1(c) are deleted and references to "Kurt Knutson"
and "(Telecopy No. (816) 979-7561)", respectively, are inserted in lieu
thereof.

         (d)     The definition of "Revolving Credit Commitment" contained in
Exhibit 1 is deleted and the following definition is inserted in lieu thereof:

                 "Revolving Credit Commitment" shall mean Fifty Million Dollars
                 ($50,000,000.00).

         3.      Amendments To Revolving Note.  The Revolving Note is amended
as follows:

         (a)     References on the first page to "$7,000,000.00" or "Seven
Million Dollars ($7,000,000.00)" are deleted and references to "$50,000,000.00"
and "Fifty Million Dollars ($50,000,000.00)", respectively, are inserted in
lieu thereof.

         (b)     The portion of the definition of "Applicable Margin" contained
within (a) (v) is deleted and the following provision is inserted in lieu
thereof:

                 (v)      equal to or greater than 5.00 to 1.00, 50 basis
                 points.

         (c)     The definition of "Maturity" as contained in paragraph 1 on
page 3, is deleted and the following provision is inserted in lieu thereof:

                 (1)      "Maturity" means June 30, 1999.

         4.      Cross-Referenced Terms.  Each reference in the Loan Documents
to the Credit Agreement or the Revolving Note shall henceforth refer to the
Credit Agreement or the Revolving Note as amended hereby.

         5.      Conditions Precedent.  On or prior to the date hereof, the
Bank shall have received the following, each of which shall be in form and
substance satisfactory to the Bank:





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         (a)     evidence of the authority of the Borrowing Group to enter into
the transactions contemplated hereby; and

         (b)     all other documents, opinions and items as the Bank may
request.

         6.      Ratification; Estoppel.  The terms, conditions, covenants and
provisions of the Credit Agreement, the Revolving Note, the Term Loan Guaranty,
the Revolving Loan Guaranty and the Loan Documents, as amended hereby, are
ratified and confirmed by the Borrowing Group in all respects as of the date
hereof.

         7.      No Impairment.  Nothing in this Agreement shall be deemed to
or shall in any manner prejudice or impair the Loan Documents held by the Bank
for the indebtedness evidenced by the Notes arising under the Credit Agreement.
This Agreement shall not be deemed to be nor shall it constitute any
alteration, waiver, annulment or variation of any of the terms, covenants and
provisions of or any rights, powers or remedies under any Loan Document, except
as expressly set forth herein.

         8.      Law.  This Agreement shall be a contract made under, governed
by and construed in accordance with, the internal laws of the State of
Missouri.

         9.      Survival of Representations.  All covenants, representations
and warranties made by Borrowing Group herein and in the Credit Agreement and
any Loan Document shall survive the delivery of this Agreement and the
effective date hereof and shall continue in effect until the Notes are fully
repaid and all obligations thereunder completely performed.

         10.     Successors.  This Agreement shall be binding upon and inure to
the benefit of the parties hereto and their respective successors and assigns;
provided, however, that the Borrowing Group shall not assign this Agreement or
the Credit Agreement or any of their rights or duties hereunder or thereunder,
without the prior written consent of the Bank.

         11.     Captions.  The section headings and captions in this Agreement
are for convenience only and shall not affect the construction thereof.

         12.     Terms and Definitions.  Terms capitalized herein which are not
defined are to be accorded the meaning such terms possess in the Credit
Agreement.

         13.     Waiver of Claims and Defenses.  The Borrowing Group
acknowledges, as of the date hereof, their obligation for full payment of
amounts outstanding under the Notes and hereby waive any and all claims or
defenses, known or unknown, existing as of the date hereof, which would
diminish their obligation of repayment





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under the Notes, the Term Loan Guaranty, or the Revolving Loan Guaranty or
which in any manner arise out of or relate to any Loan Document.

         14.     NO ORAL CREDIT AGREEMENT.  ORAL AGREEMENTS OR COMMITMENTS TO
LOAN MONEY, EXTEND CREDIT OR TO FORBEAR FROM ENFORCING REPAYMENT OF A DEBT
INCLUDING PROMISES TO EXTEND OR RENEW SUCH DEBT ARE NOT ENFORCEABLE.  TO
PROTECT YOU (BORROWER(S)) AND US (CREDITOR) FROM MISUNDERSTANDING OR
DISAPPOINTMENT, ANY AGREEMENTS WE REACH COVERING SUCH MATTERS ARE CONTAINED IN
THIS WRITING, WHICH IS THE COMPLETE AND EXCLUSIVE STATEMENT OF THE AGREEMENT
BETWEEN US, EXCEPT AS WE MAY LATER AGREE IN WRITING TO MODIFY IT.

         IN WITNESS WHEREOF, the Borrowing Group and the Bank have executed
this Agreement as of the date and year first above written.

                                         BORROWING GROUP:

ATTEST:                                  REPUBLIC GROUP INCORPORATED


/s/ JANEY L. RIFE                        By: /s/ DOYLE R. RAMSEY
-----------------------------               ----------------------------------
Secretary                                   Name:  Doyle R. Ramsey
(Corporate Seal)                            Title:  Vice President Finance


ATTEST:                                  REPUBLIC PAPERBOARD COMPANY


/s/ JANEY L. RIFE                        By: /s/ DOYLE R. RAMSEY
-----------------------------               ----------------------------------
Secretary                                   Name:  Doyle R. Ramsey
(Corporate Seal)                            Title:  Vice President Finance


ATTEST:                                  REPUBLIC PAPERBOARD COMPANY
                                           OF WEST VIRGINIA


/s/ JANEY L. RIFE                        By: /s/ DOYLE R. RAMSEY
-----------------------------               ----------------------------------
Secretary                                   Name:  Doyle R. Ramsey
(Corporate Seal)                            Title:  Vice President Finance


ATTEST:                                  REPUBLIC GYPSUM COMPANY


/s/ JANEY L. RIFE                        By: /s/ DOYLE R. RAMSEY
-----------------------------               ----------------------------------
Secretary                                   Name:  Doyle R. Ramsey
(Corporate Seal)                            Title:  Vice President Finance





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                                         BANK:

                                         NATIONSBANK, N.A.
                                         Successor by merger to
                                         BOATMEN'S FIRST NATIONAL BANK
                                            OF KANSAS CITY


                                         By: /s/ KURT KNUTSON
                                            ----------------------------------
                                            Kurt Knutson
                                            Vice President





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<PAGE>   7
STATE OF KANSAS           )
                          )       SS.
COUNTY OF RENO            )

         On this 28th day of April, 1998, before me, personally appeared Doyle
R. Ramsey, who, being first duly sworn and known by me to be the person who
executed this Fourth Amendment To Loan Documents, did say that he has full
power and authority to execute this instrument in the name of Republic Group
Incorporated and is doing so with the full knowledge and consent of the Board
of Directors of said Corporation and that the execution of said instrument is
his own free act and deed on behalf of said Corporation.


                                             /s/ CHERRI L. HARRIS
                                             ---------------------------------
                                             Notary Public


My Commission Expires:

February 27, 1999
-------------------------                                                     




STATE OF KANSAS           )
                          )       SS.
COUNTY OF RENO            )

         On this 28th day of April, 1998, before me, personally appeared Doyle
R. Ramsey, who, being first duly sworn and known by me to be the person who
executed this Fourth Amendment To Loan Documents, did say that he has full
power and authority to execute this instrument in the name of Republic
Paperboard Company and is doing so with the full knowledge and consent of the
Board of Directors of said Corporation and that the execution of said
instrument is his own free act and deed on behalf of said Corporation.

                                             /s/ CHERRI L. HARRIS
                                             ---------------------------------
                                             Notary Public


My Commission Expires:

February 27, 1999
-------------------------                                                     






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                                    Page 12
<PAGE>   8
STATE OF KANSAS           )
                          )       SS.
COUNTY OF RENO            )

         On this 28th day of April, 1998, before me, personally appeared Doyle
R. Ramsey, who, being first duly sworn and known by me to be the person who
executed this Fourth Amendment To Loan Documents, did say that he has full
power and authority to execute this instrument in the name of Republic
Paperboard Company of West Virginia and is doing so with the full knowledge and
consent of the Board of Directors of said Corporation and that the execution of
said instrument is his own free act and deed on behalf of said Corporation.

                                             /s/ CHERRI L. HARRIS
                                             ---------------------------------
                                             Notary Public


My Commission Expires:

February 27, 1999
-------------------------                                                     






STATE OF KANSAS           )
                          )       SS.
COUNTY OF RENO            )

         On this 28th day of April, 1998, before me, personally appeared Doyle
R. Ramsey, who, being first duly sworn and known by me to be the person who
executed this Fourth Amendment To Loan Documents, did say that he has full
power and authority to execute this instrument in the name of Republic Gypsum
Company and is doing so with the full knowledge and consent of the Board of
Directors of said Corporation and that the execution of said instrument is his
own free act and deed on behalf of said Corporation.




                                             /s/ CHERRI L. HARRIS
                                             ---------------------------------
                                             Notary Public


My Commission Expires:

February 27, 1999
-------------------------                                                     




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STATE OF MISSOURI         )
                          )       SS.
COUNTY OF JACKSON         )

         On this 28th day of April, 1998, before me, personally appeared Kurt
Knutson, who being first duly sworn and known by me to be the person who
executed this Fourth Amendment To Loan Documents, did say that he has full
power and authority to execute this instrument in the name of NationsBank, N.A.
and is doing so as his own free act and deed on behalf of said Bank.

         In Witness Whereof, I have hereunto set my hand and affixed my
notarial seal on the day and year last above written.

                                             /s/ NANCY LAUBER
                                             ---------------------------------
                                             Notary Public


My Commission Expires:

September 4, 2001
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