<PAGE>   1

                                                                   EXHIBIT 4.1.8

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<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.

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<CAPTION>
<S>                                                                                <C>                         <C>
Dated:
       ---------------------------------                                           -------------------------------------------------
                                                                                   Title:                      Title:
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<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.

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<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
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(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.

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<S>                                                                             <C>
Dated,
       -------------------------------------
                  In presence of
                                                                                ----------------------------------------------------
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