Document:

<PAGE>   1

                                                                   EXHIBIT 4.1.8

<TABLE>
<CAPTION>
<S>                                                        <C>                                                     <C>
         Number                                            [GRAPHIC OF EAGLE]                                        Shares
           G-
             --                                                                                                    ------------
</TABLE>

                          NEXTLINK Communications, Inc.

          Series G Cumulative Convertible Participating Preferred Stock

                            $.01 Par Value Per Share

         THIS IS TO CERTIFY THAT __________________________ is the owner of
__________________________________ fully paid and non-assessable Shares of the
above Corporation transferable only on the books of the Corporation by the
holder hereof in person or by a duly authorized Attorney upon surrender of this
Certificate properly endorsed.

         IN WITNESS WHEREOF, the said Corporation has caused this Certificate to
be signed by its duly authorized officers and to be sealed with the Seal of the
Corporation.

<TABLE>
<CAPTION>
<S>                                                                                <C>                         <C>
Dated:
       ---------------------------------                                           -------------------------------------------------
                                                                                   Title:                      Title:
</TABLE>

<PAGE>   2

THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER
THE SECUIRITIES ACT OF 1933 OR THE SECURITIES LAWS OF ANY STATE AND MAY NOT BE
SOLD OR OTHERWISE DISPOSED OF EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION
STATEMENT UNDER SUCH ACT AND APPLICABLE STATE SECURITIES LAWS OR AN APPLICABLE
EXEMPTION TO THE REGISTRATION REQUIREMENTS OF SUCH ACT OR SUCH LAWS.

THE SALE, PLEDGE, TRANSFER, ASSIGNMENT OR OTHER DISPOSITION OF THE SHARES
REPRESENTED BY THIS CERTIFICATE IS RESTRICTED BY AND SUBJECT TO THE PROVISIONS
OF A STOCK PURCHASE AGREEMENT DATED AS OF DECEMBER 7, 1999, A COPY OF WHICH IS
AVAILABLE UPON REQUEST FOR INSPECTION AT THE OFFICES OF THE CORPORATION. ANY
SUCH REQUEST SHOULD BE ADDRESSED TO THE SECRETARY OF THE CORPORATION.

THE CORPORATION IS AUTHORIZED TO ISSUE MORE THAN ONE CLASS OR SERIES OF STOCK. A
COPY OF THE PREFERENCES, POWERS, QUALIFICATIONS AND RIGHTS OF EACH CLASS AND
SERIES WILL BE FURNISHED BY THE CORPORATION UPON WRITTEN REQUEST AND WITHOUT
CAUSE.

         The following abbreviations, when used in the inscription on the face
of this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations. Additional abbreviations may also
be used though not in the list.

<TABLE>
<S>      <C>               <C>                                         <C>
         TEN COM  --       as tenants in common                        UNIF GIFT MIN ACT --..............Custodian.......... (Minor)
         TEN ENT  --       as tenants by the entireties                         under Uniform Gifts to Minors Act........... (State)
         JT TEN   --       as joint tenants with right of survivorship
                           and not as tenants in common
For value received, the undersigned hereby sells, assigns and transfers unto                     PLEASE INSERT SOCIAL  SECURITY OR
                                                                                                 OTHER IDENTIFYING NUMBER OF
                                                                                                 ASSIGNEE
</TABLE>

--------------------------------------------------------------
(PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

--------------------------------------------------------------------------------
--------------------------------------------------------------------- --- Shares
represented by the within Certificate, and hereby irrevocably constitutes
and appoints ---------------------------------------------
--------------------------------------------- Attorney to transfer the said
shares on the books of the within-named Corporation with full power of
substituton in the premises.

<TABLE>
<S>                                                                             <C>
Dated,
       -------------------------------------
                  In presence of
                                                                                ----------------------------------------------------
</TABLE><PAGE>   1

                                                                   EXHIBIT 4.1.9

<TABLE>
<CAPTION>
<S>                                                        <C>                                                     <C>
         Number                                            [GRAPHIC OF EAGLE                                         Shares
           H-
             --                                                                                                    ------------
</TABLE>

                          NEXTLINK Communications, Inc.

               Series H Convertible Participating Preferred Stock

                            $.01 Par Value Per Share

         THIS IS TO CERTIFY THAT __________________________ is the owner of
__________________________________ fully paid and non-assessable Shares of the
above Corporation transferable only on the books of the Corporation by the
holder hereof in person or by a duly authorized Attorney upon surrender of this
Certificate properly endorsed.

         IN WITNESS WHEREOF, the said Corporation has caused this Certificate to
be signed by its duly authorized officers and to be sealed with the Seal of the
Corporation.

<TABLE>
<CAPTION>
<S>                                                                                <C>                         <C>
Dated:
       ---------------------------------                                           -------------------------------------------------
                                                                                   Title:                      Title:
</TABLE>

<PAGE>   2

THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER
THE SECUIRITIES ACT OF 1933 OR THE SECURITIES LAWS OF ANY STATE AND MAY NOT BE
SOLD OR OTHERWISE DISPOSED OF EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION
STATEMENT UNDER SUCH ACT AND APPLICABLE STATE SECURITIES LAWS OR AN APPLICABLE
EXEMPTION TO THE REGISTRATION REQUIREMENTS OF SUCH ACT OR SUCH LAWS.

THE SALE, PLEDGE, TRANSFER, ASSIGNMENT OR OTHER DISPOSITION OF THE SHARES
REPRESENTED BY THIS CERTIFICATE IS RESTRICTED BY AND SUBJECT TO THE PROVISIONS
OF A STOCK PURCHASE AGREEMENT DATED AS OF DECEMBER 7, 1999, A COPY OF WHICH IS
AVAILABLE UPON REQUEST FOR INSPECTION AT THE OFFICES OF THE CORPORATION. ANY
SUCH REQUEST SHOULD BE ADDRESSED TO THE SECRETARY OF THE CORPORATION.

THE CORPORATION IS AUTHORIZED TO ISSUE MORE THAN ONE CLASS OR SERIES OF STOCK. A
COPY OF THE PREFERENCES, POWERS, QUALIFICATIONS AND RIGHTS OF EACH CLASS AND
SERIES WILL BE FURNISHED BY THE CORPORATION UPON WRITTEN REQUEST AND WITHOUT
CAUSE.

         The following abbreviations, when used in the inscription on the face
of this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations. Additional abbreviations may also
be used though not in the list.

<TABLE>
<S>      <C>               <C>                                         <C>
         TEN COM  --       as tenants in common                        UNIF GIFT MIN ACT --..............Custodian.......... (Minor)
         TEN ENT  --       as tenants by the entireties                         under Uniform Gifts to Minors Act........... (State)
         JT TEN   --       as joint tenants with right of survivorship
                           and not as tenants in common
For value received, the undersigned hereby sells, assigns and transfers unto                     PLEASE INSERT SOCIAL  SECURITY OR
                                                                                                 OTHER IDENTIFYING NUMBER OF
                                                                                                 ASSIGNEE
</TABLE>

--------------------------------------------------------------
(PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

--------------------------------------------------------------------------------
--------------------------------------------------------------------- --- Shares
represented by the within Certificate, and hereby irrevocably constitutes
and appoints ---------------------------------------------
--------------------------------------------- Attorney to transfer the said
shares on the books of the within-named Corporation with full power of
substituton in the premises.

<TABLE>
<S>                                                                             <C>
Dated,
       -------------------------------------
                  In presence of
                                                                                ----------------------------------------------------
</TABLE><PAGE>   1

                                                                   EXHIBIT 4.2.4

                                                                  EXECUTION COPY

===============================================================================

                          NEXTLINK Communications, Inc.

                                     Issuer

                                       and

                             NEXTLINK Capital, Inc.

                                       and

                              NM ACQUISITION CORP.
                  (to be the successor by merger to the Issuer)

                                       and

                     UNITED STATES TRUST COMPANY OF NEW YORK
                                     Trustee

                    ----------------------------------------

                                      THIRD
                             SUPPLEMENTAL INDENTURE
                            Dated as of June 15, 2000

                                    Amending

                                    INDENTURE
                           Dated as of April 25, 1996

                    ----------------------------------------

                                  $350,000,000

                              12 1/2 % Senior Notes
                               Due April 15, 2006

===============================================================================

<PAGE>   2

         THIS THIRD SUPPLEMENTAL INDENTURE, dated as of June 15, 2000 (herein
called the "Supplement"), is by and among NEXTLINK COMMUNICATIONS, INC., a
corporation organized and existing under the laws of the State of Delaware
("NEXTLINK" or the "Issuer"), having its principal office at 1505 Farm Credit
Drive, McLean, VA 22102, NEXTLINK Capital, Inc., a corporation organized and
existing under the laws of the State of Delaware, NM ACQUISITION CORP. (to be
known, following the Merger (as defined below), as NEXTLINK Communications,
Inc.), a corporation organized and existing under the laws of the State of
Delaware (the "Company"), having its principal office at 1505 Farm Credit Drive,
McLean, VA 22102, and UNITED STATES TRUST COMPANY OF NEW YORK, duly organized
and existing under the laws of the State of New York, as Trustee (herein called
the "Trustee").

                                    RECITALS

         WHEREAS, pursuant to the terms of the Indenture, dated as of April 25,
1996 (as supplemented by the First Supplemental Indenture dated as of January
31, 1997 and Second Supplemental Indenture dated as of June 3, 1998, herein
called the "Original Indenture"), between NEXTLINK (or its predecessor) and the
Trustee, $350,000,000 principal amount of 12 1/2 % Senior Notes due April 15,
2006 (herein called the "Securities") were issued; and

         WHEREAS, pursuant to the terms of that certain Amended and Restated
Agreement and Plan of Merger and Share Exchange Agreement, dated as of May 10,
2000, by and among Concentric Network Corporation, NEXTLINK, the Company, Eagle
River Investments, L.L.C. and Craig O. McCaw, NEXTLINK will be merged with and
into the Company, with the Company as the surviving corporation (the "Merger");
and

         WHEREAS, as a result of the Merger, the Company will change its name to
NEXTLINK Communications, Inc.; and

         WHEREAS, Section 901 of the Original Indenture provides for the
execution and delivery by the Issuer and, subject to the provisions of Section
903 of the Original Indenture, by the Trustee of one or more supplemental
indentures, without the consent of the Holders of the Securities, for the
purposes specified therein; and

         WHEREAS, pursuant to the provisions of Section 801 of the Original
Indenture and assuming the requirements of such Section are satisfied, NEXTLINK
is permitted to merge with and into the Company through a merger transaction in
which NEXTLINK is not the surviving corporation, and under the Original
Indenture, the Issuer and the Trustee may enter into a supplemental indenture to
evidence the succession of another Person to the Issuer and the assumption by
such successor of the covenants of the Issuer contained in the Original
Indenture, which supplement, pursuant to Section 901 of the Original Indenture,
does not require the consent of the Holders of the Securities; and

         WHEREAS, pursuant to the provisions of Section 801(b) the Company
wishes by this Supplement to evidence its succession to NEXTLINK and its
assumption of the covenants of NEXTLINK contained in the Original Indenture and
the Securities and pursuant to the

<PAGE>   3

provisions of Section 802 the Company shall succeed to, and be substituted for,
and may exercise every right and power of, NEXTLINK under the Original
Indenture; and

         WHEREAS, all things necessary to make this Supplement, when executed
and delivered by the Trustee, the valid agreement of the Company in accordance
with its terms have been done.

         NOW, THEREFORE, THIS SUPPLEMENT WITNESSETH:

         SECTION 101. Definitions. Except as otherwise expressly provided
herein, all capitalized words and terms used herein shall have the respective
meanings ascribed thereto in the Original Indenture.

         SECTION 102. Representations of the Company. The Company hereby
represents and warrants to the Trustee that as of the date hereof:

         (a) The Company is a corporation validly existing and in good standing
under the laws of the State of Delaware; and

         (b) No Default or Event of Default will result from the Merger or the
execution and delivery of this Supplement.

         SECTION 103. Assumption of Obligations. The Company hereby assumes all
of the obligations of NEXTLINK under the Original Indenture.

         SECTION 104. Construction with Original Indenture. All of the
covenants, agreements and provisions of this Supplement shall be deemed to be
and construed as part of the Original Indenture and VICE VERSA to the same
extent as if fully set forth verbatim therein and herein and shall be fully
enforceable in the manner provided in the Original Indenture. Except as provided
in this Supplement, the Original Indenture shall remain in full force and effect
and the terms and conditions thereof are hereby confirmed.

         SECTION 105. Conflict with Trust Indenture Act. If any provision hereof
limits, qualifies or conflicts with a provision of the Trust Indenture Act that
is required under such Act to be part of and govern the Original Indenture or
this Supplement, the latter provision shall control. If any provision hereof
modifies or excludes any provision of the Trust Indenture Act that may be so
modified or excluded, the latter provision shall be deemed to apply to this
Supplement as so modified or to be excluded, as the case may be.

         SECTION 106. Effect of Headings. The Section headings herein are for
convenience only and shall not affect the construction hereof.

         SECTION 107. Separability Clause. In case any provision in this
Supplement shall be invalid, illegal or unenforceable, the validity, legality
and enforceability of the remaining provisions shall not in any way be affected
or impaired thereby, it being intended that all of the provisions hereof shall
be enforceable to the full extent permitted by law.

                                       2
<PAGE>   4

         SECTION 108. Benefits of Supplement and Original Indenture. Nothing in
this Supplement or the Original Indenture or in the Securities, express or
implied, shall give to any Person other than the parties hereto and thereto and
their successors hereunder and thereunder and the Holders of Securities, any
benefit or any legal or equitable right, remedy or claim under this Supplement
or the Original Indenture. Neither this Supplement nor the Original Indenture
may be used to interpret another indenture, loan agreement or debt agreement of
the Company or any of its Subsidiaries. No such other indenture or loan or debt
agreement may be utilized to interpret this Supplement or the Original
Indenture.

         SECTION 109. GOVERNING LAW. THIS SUPPLEMENT SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

         SECTION 110. No Recourse Against Others. A director, member, managing
member officer, employee, stockholder or incorporator, as such, of the Issuer or
the Company shall not have any liability for any obligations of the Company
under this Supplement or for any claim based on, in respect or by reason of such
obligations or their creation.

         SECTION 111. Duplicate Originals. All parties may sign any number of
copies or counterparts of this Supplement. Each signed copy or counterpart shall
be an original, but all of them together shall represent the same agreement.

                            [Signature page follows]

                                       3
<PAGE>   5

         SECTION 112. Effectiveness. This Supplement shall become effective upon
consummation of the Merger and otherwise in accordance with the provisions of
Article Nine of the Original Indenture.

         IN WITNESS WHEREOF, the parties hereto have caused this Supplement to
be duly executed as of the day and year first above written.

                                           NEXTLINK COMMUNICATIONS, INC.

                                           By:/s/ Gary D. Begeman
                                              ---------------------------------
                                              Name: Gary D. Begeman
                                              Title: Senior Vice President
Attest:

/s/ Richard A. Montfort
-----------------------------------
Name: Richard A. Montfort
Title: Assistant Secretary

                                           NM ACQUISITION CORP.

                                           By:/s/ Gary D. Begeman
                                              ---------------------------------
                                              Name: Gary D. Begeman
                                              Title: Vice President

Attest:

/s/ Richard A. Montfort
-----------------------------------
Name: Richard A. Montfort
Title: Assistant Secretary

                [Signature page to Third Supplemental Indenture]

                                       4
<PAGE>   6

                                           UNITED STATES TRUST COMPANY
                                           OF NEW YORK, Trustee

                                           By:/s/ Patricia Gallagher
                                              ---------------------------------
                                              Name: Patricia Gallagher
                                              Title: Assistant Vice President

Attest:

/s/ Kevin Fox
-----------------------------------
Name: Kevin Fox
Title: Assistant Secretary

STATE OF VIRGINIA )
                  )ss.:
COUNTY OF FAIRFAX )

         On the 15th day of June, 2000, before me personally came Gary D.
Begeman, to me known, who, being by me duly sworn, did depose and say that he is
the Senior Vice President of NEXTLINK COMMUNICATIONS, INC., a Delaware
corporation, one of the corporations described in and which executed the
foregoing instrument, and duly acknowledged to me that he executed the same by
authority of the Board of Directors of said corporation.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal the
day and year first above written.

                                           /s/ Cathryn O. Lowe
                                           -------------------------------------
                                           NOTARY PUBLIC in and for the State of
                                           Virginia, residing at Fairfax, VA
                                                                ----------------
                                           My appointment expires 08-31-04
                                                                 ---------------
                                           Print Name Cathryn O. Lowe
                                                      --------------------------

                [Signature Page to Third Supplemental Indenture]

                                       5
<PAGE>   7

STATE OF VIRGINIA )
                  )ss.:
COUNTY OF FAIRFAX )

         On the 15th day of June, 2000, before me personally came Gary D.
Begeman, to me known, who, being by me duly sworn, did depose and say that he is
the Vice President of NM Acquisition Corp., a Delaware corporation, one of the
corporations described in and which executed the foregoing instrument, and duly
acknowledged to me that he/she executed the same by authority of the Board of
Directors of said corporation.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal the
day and year first above written.

                                           /s/ Cathryn O. Lowe
                                           -------------------------------------
                                           NOTARY PUBLIC in and for the State of
                                           Virginia, residing at Fairfax, VA
                                                                ----------------
                                           My appointment expires 08-31-04
                                                                 ---------------
                                           Print Name Cathryn O. Lowe
                                                      --------------------------

STATE OF NEW YORK  )
                   ) ss.:
COUNTY OF NEW YORK )

         On the 15th day of June, 2000, before me personally came Patricia
Gallagher, to me known, who, being by me duly sworn, did depose and say that
he/she is Asst. Vice President of UNITED STATES TRUST COMPANY OF NEW YORK, one
of the corporations described in and which executed the foregoing instrument,
and duly acknowledged to me that he/she signed his/her name thereto by authority
of the By-Laws of said corporation.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal the
day and year first above written.

                                           /s/ Christine C. Collins
                                           ------------------------------------
                                           NOTARY PUBLIC in and for the State
                                           of New York, residing at
                                           3219 Eastchester Rd., BxNY10469
                                           ------------------------------------
                                           My appointment expires March 30,
                                           2002
                                               --------------------------------
                                           Print Name Christine C. Collins
                                                     --------------------------

                                           /s/ Christine C. Collins
                                           ------------------------------------
                                           Notary Public, State of New York
                                           No. 03-4624735
                                              ---------------------------------
                                           Qualified in Bronx             County
                                                        ------------------
                                           Certificate Filed in New York  County
                                                                ----------
                                           Commission Expires March 30, 2002
                                                              -----------------

                [Signature page to Third Supplemental Indenture]

                                       6

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