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                                                                     Exhibit 2.5


                                 AMENDMENT NO. 4
                                       TO
                          AGREEMENT AND PLAN OF MERGER


         AMENDMENT dated as of September 7, 2000 (this "Amendment") among Paging
Network, Inc., a Delaware corporation ("PageNet"), Arch Communications Group,
Inc., a Delaware corporation ("Arch"), and St. Louis Acquisition Corp., a
Delaware corporation and a wholly owned subsidiary of Arch ("Merger Sub").

         WHEREAS, PageNet, Arch and Merger Sub have previously entered into that
certain Agreement and Plan of Merger dated as of November 7, 1999, as amended on
January 7, 2000 and as further amended on May 10, 2000 and July 23, 2000, among
PageNet, Arch and Merger Sub (as amended, the "Merger Agreement"); and

         WHEREAS, the respective Boards of Directors of PageNet, Arch and Merger
Sub have determined that it is in the best interests of PageNet or Arch, as the
case may be, and its respective stockholders to amend the Merger Agreement as
hereinafter set forth and have duly approved this Amendment and authorized its
execution and delivery.

         NOW, THEREFORE, the parties hereto agree as follows:

         1. All capitalized terms used herein, unless otherwise defined herein,
shall have the meanings given them in the Merger Agreement, and each reference
in the Merger Agreement to "this Agreement", "hereof", "herein", "hereunder" or
"hereby" and each other similar reference shall be deemed to refer to the Merger
Agreement as amended hereby. All references to the Merger Agreement in any other
agreement between PageNet and Arch relating to the transactions contemplated by
the Merger Agreement shall be deemed to refer to the Merger Agreement as amended
hereby.

         2. Section 4.1(a) of the Merger Agreement is hereby deleted in its
entirety and replaced with the following:

         "(a) Merger Consideration. Each share of common stock, par value $0.01
per share, of PageNet (each, a "PageNet Share") issued and outstanding
immediately prior to the Effective Time (excluding PageNet Shares (collectively,
"Excluded PageNet Shares") that are owned by Arch, Merger Sub or any direct or
indirect, wholly owned subsidiary of Arch or Merger Sub (collectively, the "Arch
Companies")), shall be converted into and become exchangeable for 0.04796505 of
a share (the "Exchange Ratio") of common stock, par value $0.01 per share, of
Arch (the "Arch Common Stock"), subject to adjustment as provided in Section 4.4
(the "Merger Consideration"). At the Effective Time, all PageNet Shares shall no
longer be outstanding, shall be canceled and retired and shall cease to exist,
and each certificate (a "Certificate") formerly


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representing any of such PageNet Shares (other than Excluded PageNet Shares)
shall thereafter represent only the right to receive the Merger Consideration
and the right, if any, to receive a distribution or dividend pursuant to
Section 4.2(b)(i), in each case without interest, or to vote pursuant to
Section 4.2(b)(ii)."

         3. The Merger Agreement, as amended hereby, is hereby ratified and
confirmed in all respects.



                                      * * *


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         IN WITNESS WHEREOF, this Amendment has been duly executed and delivered
by the duly authorized officers of the parties to this Amendment as of the date
first written above.


                                       PAGING NETWORK, INC.


                                       By: /s/ JOHN P. FRAZEE, JR.
                                           -------------------------------------
                                           Name:  John P. Frazee, Jr.
                                           Title: Chairman of the Board and
                                                  Chief Executive Officer


                                       ARCH COMMUNICATIONS GROUP, INC.


                                       By:  /s/ J. ROY POTTLE
                                           -------------------------------------
                                            Name:  J. Roy Pottle
                                            Title: Executive Vice President
                                                   and Chief Financial Officer


                                       ST. LOUIS ACQUISITION CORP.


                                       By: /s/ J. ROY POTTLE
                                           -------------------------------------
                                           Name:  J. Roy Pottle
                                           Title: Executive Vice President and
                                                  Chief Financial Officer