Document:

<PAGE>

                                                                 Exhibit 10.15.4

                             TERMINATION AGREEMENT

By this private document, on one side, Dialdata Internet Systems S.A., company
incorporated according to the Brazilian Law, with its headquarters at Rua
Bandeira Paulista, 716, 1st floor, in the City of Sao Paulo, State of Sao Paulo,
herein by its undersigned legal representatives, hereinafter referred to as
DIALDATA and, in the other side Mr. Antonio Alberto Valente Tavares, Portuguese
citizen, married, businessman, bearer of Identity Card RNE W 429.270.997-15,
hereinafter simply referred as TAVARES;

                                    WHEREAS:

    (a)   Parties signed a Retention Agreement, in which TAVARES was hired to
       render services to DIALDATA as Chief Executive Officer, pursuant
       DIALDATA's bylaws, dated December 29,1998;

    (b)   TAVARES accepted, in May 12, 1999, an offer made by VIA NET.WORKS,
       Inc., (the ultimate parent company of DIALDATA) to change his position as
       CEO of DIALDATA to Vice President Latin American Region of VIA;

NOW, THEREFORE, in consideration of the mentioned agreements mentioned above,
parties hereto agree as follows:

Section 01.  Parties hereto agree, amicably, to terminate the Retention
Agreement mentioned in WHEREAS (a), finishing the obligations of each one of
then, according to the Retention Agreement.
<PAGE>

Section 02.  Tavares will formally remain as CEO of DIALDATA until the
shareholders of DIALDATA have appointed a substitute pursuant DIALDATA's bylaws
and TAVARES agrees to perform his duties as CEO of DIALDATA, during this period.

Section 03. As compensation for the performance of the duties as CEO of
DIALDATA, by TAVARES, Parties agree that TAVARES will have the right to receive
the maximum annual bonus, as set forth in Section 03. Second Paragraph of the
Retention Agreement, calculated "pro rata", for the period from January 1, 1999
through May 11, 1999.  The Parties agree that for the period from May 11, 1999
through December 31, 1999 TAVARES will have the right to receive the maximum
annual bonus of up to 60% of his base annual salary, which shall be determined
according to his performance of his duties and goals as Vice President Latin
American Region.  If TAVARES remains employed by VIA NET.WORKS, Inc. after
December 31, 1999, he will have the right to receive a maximum annual bonus of
up to 50% of his base annual salary.

Section 04. All bonus payments for any periods beginning May 11, 1999 made
pursuant to Section 03. above, shall be made by VIA NET.WORKS, Inc.

Section 05. In consideration of the premises herein contained, the parties
hereto give each other full and general release as to the past or present
relationship between them and commit themselves not to claim any right or
interest deriving therefrom or in connection thereto.

IN WITNESS WHEREOF, the parties hereto cause this Agreement in two counterparts
of same content and for the same purpose, in the presence of 2 (two) undersigned
witnesses dated as of October 21, 1999.

                                       2
<PAGE>

          DIALDATA S.A. INTERNET SYSTEMS

       By:     /s/ Antonio Alberto Valente Tavares
       Name:   Antonio Alberto Valente Tavares
       Title:  Vice President Latin America Region

       ANTONIO ALBERTO VALENTE TAVARES

       VIA NET.WORKS, INC.

       By:     /s/ Matt Nydell
       Name:   Matt Nydell
       Title:  VP & General Counsel

       Witnessed by: [signature illegible]
                     [signature illegible]

                                       3<PAGE>

               INCORPORATED UNDER THE LAWS OF THE STATE OF NEVADA

                                  COMMON STOCK

                           ENTERPRISES SOLUTIONS, INC.

NUMBER                                         SHARES
       --------------------                           --------------------------

                                                      CUSIP 29381G 1 2

                                                       SEE REVERSE FOR
                                                     CERTAIN DEFINITIONS

--------------------------------------------------------------------------------
THIS CERTIFIES that

is the owner of
--------------------------------------------------------------------------------

FULLY PAID AND NON-ASSESSABLE SHARES, OF THE PAR VALUE OF $.001 PER SHARE, OF
THE COMMON STOCK OF

                           ENTERPRISES SOLUTIONS, INC.

                              CERTIFICATE OF STOCK

transferable on the books of the Corporation by the holder thereof, in person or
by duly authorized attorney upon surrender of this Certificate properly
endorsed. This Certificate and the shares represented hereby are issued under
and shall be held subject to all the provisions of the Articles of Incorporation
of the Corporation and the By-laws now or hereafter amended.

         This Certificate is not valid unless countersigned by the Transfer
Agent and registered by the Registrar.

         Witness the seal of the Corporation and the facsimile signatures of its
duly authorized officers.

Dated

                                     [seal]

/s/                                                /s/
  -------------------                                 -----------------
  Secretary/Treasuer                                      President

COUNTERSIGNED AND REGISTERED:
         AMERICAN STOCK TRANSFER & TRUST COMPANY
                     (NEW YORK, N.Y.)      TRANSFER AGENT
BY                                         AND REGISTRAR

                                          AUTHORIZED SIGNATURE

<PAGE>

                           ENTERPRISES SOLUTIONS, INC.

         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.

<TABLE>
<CAPTION>

<S>           <C>    <C>                                    <C>                    <C>
TEN COM         --   as tenants in common                     UNIF GIFT MIN ACT -- _______________ Custodian______________
TEN ENT         --   as tenants by the entireties                                      (Cust)                  (Minor)
JT TEN               as joint tenants with rights of
                     survivorship and not as tenants in                             under Uniform Gifts to Minors Act
                     common
                                                                                    _________________________________
                                                                                                  (State)

     Additional abbreviations may also be used though not in the above list.

         For Value received ______________________________________________________  hereby sell, assign and transfer unto

PLEASE INSERT SOCIAL SECURITY OR OTHER
IDENTIFYING NUMBER OF ASSIGNEE

|---------------------------------------|
|                                       |
|---------------------------------------|-------------------------------------------------------------------------------------------

------------------------------------------------------------------------------------------------------------------------------------
                    Please print or typewrite name, address and relationship of Assignee and number of shares

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

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

---------------------------------------------------------------------------------------------------------------------------- shares
represented by the within Certificate and, if said number of shares shall not be all the shares represented by the within
Certificate, that a new Common Stock Certificate for the shares not transferred be registered in the name of the undersigned, and

------------------------------------------------------------------------------------------------------------------------------------
is hereby irrevocably constituted and appointed as Attorney to transfer such shares as aforesaid on the books of the Corporation,
with full power of substitution in the premises.

DATED
     -------------------------------------------------------------------------------------------------------------------------------

                                                                                          ------------------------------------------
                                                                                             NOTICE The signature to this assignment
                                                                                          must correspond with the name as written
                                                                                          upon the face of the Certificate, in every
                                                                                          particular, without alteration or
                                                                                          enlargement, or any change whatever.

Signature(s) Guaranteed:

--------------------------------------------------------------------------------
THE SIGNATURE(S) MUST BE GUARANTEED BY AN ELIGIBLE GUARANTOR INSTITUTION (BANKS,
STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONS AND CREDIT UNIONS WITH MEMBERSHIP
IN AN APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM), PURSUANT TO S.E.C. RULE
17Ad.15.

</TABLE>

KEEP THIS CERTIFICATE IN A SAFE PLACE. IF IT IS LOST, STOLEN, MUTILATED OR
DESTROYED, THE CORPORATION WILL REQUIRE A BOND OF INDEMNITY AS A CONDITION TO
THE ISSUANCE OF A REPLACEMENT CERTIFICATE.<PAGE>

                                                                     Exhibit 4.5

                               State of Delaware                    PAGE 1

                       Office of the Secretary of State

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

     I, EDWARD J. FREEL, SECRETARY OF STATE OF THE STATE OF DELAWARE, DO HEREBY
CERTIFY THE ATTACHED IS A TRUE AND CORRECT COPY OF THE CERTIFICATE OF AMENDMENT
OF "BLUE RHINO CORPORATION", FILED IN THIS OFFICE ON THE TWENTY-FIRST DAY OF
DECEMBER, A.D. 1999, AT 1 O'CLOCK P.M.

     A FILED COPY OF THIS CERTIFICATE HAS BEEN FORWARDED TO THE NEW CASTLE
COUNTY RECORDER OF DEEDS.

                                    [SEAL]

                                    /s/ Edward J. Freel
                      [SEAL]        -------------------------------------------
                                    Edward J. Freel, Secretary of State

                                    AUTHENTICATION:    0156362

                                              DATE:    12-22-99
<PAGE>

                           CERTIFICATE OF AMENDMENT
                                    TO THE
                          SECOND AMENDED AND RESTATED
                         CERTIFICATE OF INCORPORATION
                                      OF
                            BLUE RHINO CORPORATION
                           (a Delaware corporation)

Pursuant to Section 242 of the General Corporation Law of the State of Delaware,
the Second Amended and Restated Certificate of Incorporation of Blue Rhino
Corporation, a Delaware corporation (the "Certificate of Incorporation"), filed
with the Secretary of State of Delaware on May 19, 1998, is hereby amended as
follows:

          Article FOURTH is hereby amended to read in its entirety as follows:

          FOURTH: The total number of shares of stock (common and preferred)
which the corporation shall have authority to issue is Forty Five Million
(45,000,000) shares consisting of:

          (a) Thirty Seven Million Five Hundred Thousand (37,500,000) shares of
common stock, with a par value of $.001 per share (the "Common Stock"); and

          (b) Seven Million Five Hundred Thousand (7,500,000) shares of
preferred stock, with a par value of $.001 per share (the "Preferred Stock").

          Except as otherwise required by law or expressly provided herein, the
holder of each share of Common Stock shall have one vote on each matter
submitted to a vote of the stockholders of the Corporation.

          The holders of the Common Stock shall be entitled to receive dividends
at such times and in such amounts as may be determined by the Board of Directors
of the Corporation.

          In the event of any liquidation, dissolution or winding up of the
Corporation, whether voluntary or involuntary, after payment or provision for
payment of the debts and other liabilities of the Corporation, the holders of
Common Stock, shall be entitled to share ratably in the remaining assets of the
Corporation.

          The holders of the Preferred Stock shall be entitled to such rights
and preferences as may be approved from time to time by the Board of Directors
of the Corporation as set forth in a Certificate of Designation filed pursuant
to the Delaware Law.

DATED: December 21, 1999

                               Blue Rhino Corporation

                               /s/ Mark Castaneda
                               -----------------------------
                               Mark Castaneda
                               Secretary

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