Document:

<PAGE>

                                                                 Exhibit 10.11.6

                   FOURTH AMENDMENT TO STOCK RIGHTS AGREEMENT

     THIS FOURTH AMENDMENT TO STOCK RIGHTS AGREEMENT (this "Amendment"), is made
as of this 7th day of August 2000, by and among Richard E. Ottalagana (the
"Stockholder"), PaeTec Corp., a Delaware corporation (the "Company"), PaeTec
Communications, Inc., a Delaware corporation and wholly-owned subsidiary of the
Company (the "Subsidiary"), and Arunas A. Chesonis ("Mr. Chesonis").

                                    RECITALS
                                    --------

     1.  The Company, the Subsidiary, the Stockholder and Mr. Chesonis are
parties to a Stock Rights Agreement dated July 17, 1998 (the "Original Stock
Rights Agreement"), as amended by (i) a First Amendment to Stock Rights
Agreement dated August 13, 1998, (ii) a Second Amendment to Stock Rights
Agreement dated September 30, 1998, and (iii) a Third Amendment to Stock Rights
Agreement dated February 4, 2000 (as heretofore amended, the "Stock Rights
Agreement").

     2.  The Company has filed a registration statement with the Securities and
Exchange Commission covering the initial public offering of the Company's common
stock (the "IPO") under the Securities Act of 1933, as amended.

     3. The parties hereto wish to amend the Stock Rights Agreement, effective
on the date and at the time of the initial closing of the IPO (the "IPO Closing
Time"), to provide for the termination of the "co-sale rights" granted to the
Stockholder in the Original Stock Rights Agreement.

                                   AGREEMENT
                                   ---------

     In consideration of the premises and of the terms and conditions herein
contained, the parties hereto mutually agree as follows:

     1.  Defined Terms.  All capitalized terms used in this Amendment without
         -------------
definition shall have the meanings given to such terms in the Stock Rights
Agreement.

     2.  Termination of "Co-Sale Rights."  The parties hereto agree that, upon
         --------------------------------
consummation of the IPO, the Stock Rights Agreement shall be amended to
terminate the "co-sale rights" granted to Stockholder in Section 7 of the Stock
Rights Agreement.  Effective as of the IPO Closing Time, Section 7 of the Stock
Rights Agreement shall be deleted in its entirety and replaced with the
following:

         "7.  [RESERVED]"
<PAGE>

If the IPO is not consummated, Section 7 of the Stock Rights Agreement shall not
be amended hereby and such section shall remain in full force and effect.

     3.  Miscellaneous.
         -------------

         3.1.  Successors and Assigns.  This Amendment shall be binding upon and
               ----------------------
shall inure to the benefit of the parties and their respective successors and
assigns.

         3.2 Governing Law. This Amendment shall be governed by, and construed
             -------------
and enforced in accordance with, the laws of the State of New York, except that
if any provision would be illegal, invalid or unenforceable under such laws in
connection with a suit or proceeding validly instituted in another jurisdiction,
the laws of such other jurisdiction shall govern insofar as is necessary to
sustain the legality, validity or enforceability of such provision or any part
of such provision.

         3.3  Captions.  Captions to the sections in this Amendment are for the
              --------
convenience of the parties only and shall not affect the meaning or
interpretation of this Amendment.

         3.4  Counterparts.  This Amendment may be executed in one or more
              ------------
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

         3.5 Additional Documents. Each party hereto agrees to execute any and
             --------------------
all documents, instruments, certificates and communications deemed to be
necessary or advisable by the Company to effectuate the purposes of this
Amendment.

                            [signature page follows]

                                       2
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed and delivered this
Amendment with full force and effect as of the day and year first written above.

                              PAETEC CORP.

                              By:   /s/ Arunas A. Chesonis
                                    ---------------------------------
                              Name: Arunas A. Chesonis
                              Its:  CEO, Chairman and President

                              PAETEC COMMUNICATIONS, INC.

                              By:   /s/ Arunas A. Chesonis
                                    ---------------------------------
                              Name: Arunas A. Chesonis
                              Its:  CEO, Chairman and President

                                    /s/ Richard E. Ottalagana
                                    ---------------------------------
                                        Richard E. Ottalagana

                                    /s/ Arunas A. Chesonis
                                    ---------------------------------
                                        Arunas A. Chesonis<PAGE>

                                                                 Exhibit 10.12.6

                   FOURTH AMENDMENT TO STOCK RIGHTS AGREEMENT

     THIS FOURTH AMENDMENT TO STOCK RIGHTS AGREEMENT (this "Amendment"), is made
as of this 7th day of August 2000, by and among Richard J. Padulo (the
"Stockholder"), PaeTec Corp., a Delaware corporation (the "Company"), PaeTec
Communications, Inc., a Delaware corporation and wholly-owned subsidiary of the
Company (the "Subsidiary"), and Arunas A. Chesonis ("Mr. Chesonis").

                                    RECITALS
                                    --------

     1.  The Company, the Subsidiary, the Stockholder and Mr. Chesonis are
parties to a Stock Rights Agreement dated July 17, 1998 (the "Original Stock
Rights Agreement"), as amended by (i) a First Amendment to Stock Rights
Agreement dated August 13, 1998, (ii) a Second Amendment to Stock Rights
Agreement dated September 30, 1998, and (iii) a Third Amendment to Stock Rights
Agreement dated February 4, 2000 (as heretofore amended, the "Stock Rights
Agreement").

     2.  The Company has filed a registration statement with the Securities and
Exchange Commission covering the initial public offering of the Company's common
stock (the "IPO") under the Securities Act of 1933, as amended.

     3. The parties hereto wish to amend the Stock Rights Agreement, effective
on the date and at the time of the initial closing of the IPO (the "IPO Closing
Time"), to provide for the termination of the "co-sale rights" granted to the
Stockholder in the Original Stock Rights Agreement.

                                   AGREEMENT
                                   ---------

     In consideration of the premises and of the terms and conditions herein
contained, the parties hereto mutually agree as follows:

     1.  Defined Terms.  All capitalized terms used in this Amendment without
         -------------
definition shall have the meanings given to such terms in the Stock Rights
Agreement.

     2.  Termination of "Co-Sale Rights."  The parties hereto agree that, upon
         --------------------------------
consummation of the IPO, the Stock Rights Agreement shall be amended to
terminate the "co-sale rights" granted to Stockholder in Section 7 of the Stock
Rights Agreement.  Effective as of the IPO Closing Time, Section 7 of the Stock
Rights Agreement shall be deleted in its entirety and replaced with the
following:

          "7.  [RESERVED]"

If the IPO is not consummated, Section 7 of the Stock Rights Agreement shall not
be amended hereby and such section shall remain in full force and effect.
<PAGE>

     3.  Miscellaneous.
         -------------

         3.1.  Successors and Assigns.  This Amendment shall be binding upon and
               ----------------------
shall inure to the benefit of the parties and their respective successors and
assigns.

         3.2 Governing Law. This Amendment shall be governed by, and construed
             -------------
and enforced in accordance with, the laws of the State of New York, except that
if any provision would be illegal, invalid or unenforceable under such laws in
connection with a suit or proceeding validly instituted in another jurisdiction,
the laws of such other jurisdiction shall govern insofar as is necessary to
sustain the legality, validity or enforceability of such provision or any part
of such provision.

         3.3  Captions.  Captions to the sections in this Amendment are for the
              --------
convenience of the parties only and shall not affect the meaning or
interpretation of this Amendment.

         3.4  Counterparts.  This Amendment may be executed in one or more
              ------------
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

         3.5 Additional Documents. Each party hereto agrees to execute any and
             --------------------
all documents, instruments, certificates and communications deemed to be
necessary or advisable by the Company to effectuate the purposes of this
Amendment.

                            [signature page follows]

                                       2
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed and delivered this
Amendment with full force and effect as of the day and year first written above.

                              PAETEC CORP.

                              By:   /s/ Arunas A. Chesonis
                                    --------------------------------
                              Name: Arunas A. Chesonis
                              Its:  CEO, Chairman and President

                              PAETEC COMMUNICATIONS, INC.

                              By:   /s/ Arunas A. Chesonis
                                    --------------------------------
                              Name: Arunas A. Chesonis
                              Its:  CEO, Chairman and President

                                    /s/ Richard J. Padulo
                                    --------------------------------
                                        Richard J. Padulo

                                    /s/ Arunas A. Chesonis
                                    --------------------------------
                                        Arunas A. Chesonis

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