<PAGE>

                                                                   Exhibit 10.4

                              AMENDMENT NUMBER ONE
                                       TO
                          REGISTRATION RIGHTS AGREEMENT


           This Amendment Number One, dated as of January 13, 2000, is entered
into by and among Hubco S.A., a Luxembourg societe anonyme (the "COMPANY"), and
the Persons listed on the signature pages hereto. Capitalized terms used and not
otherwise defined herein shall have the meanings ascribed thereto in the
"Registration Rights Agreement" (as defined below), as amended hereby.

                              W I T N E S S E T H :

           WHEREAS, the Company, the Carlyle entities named therein, iaxis,
Carrier1, Providence Equity Partners III L.P. ("PEP III") and Providence Equity
Operating Partners III L.P. ("PEOP III") are parties to that certain
Registration Rights Agreement, dated as of November 23, 1999 (the "REGISTRATION
RIGHTS AGREEMENT");

           WHEREAS, PEP III and PEOP III have ceased to be parties to the
Shareholders Agreement and have transferred all of their rights and obligations
thereunder to Providence Equity Hubco (Cayman) L.P.;

           WHEREAS, the Carlyle entities except for Carlyle Hubco International
Partners, L.P. ("CARLYLE CAYMAN") have ceased to be parties to the Shareholders
Agreement and have transferred all of their rights and obligations thereunder to
Carlyle Cayman;

           WHEREAS, the Company and the parties have agreed to amend the
Registration Rights Agreement as provided herein;

           NOW, THEREFORE, in consideration of the terms and conditions set
forth herein and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged by the parties hereto, the parties
to this Amendment, intending to be legally bound, hereby agree as follows:

           1. ADDITION OF PROVIDENCE EQUITY HUBCO (CAYMAN) L.P.; DELETION OF PEP
III AND PEOP III AND CERTAIN CARLYLE ENTITIES. (a) Providence Equity Hubco
(Cayman) L.P. is hereby added as an Investor (as defined below) and agrees to be
bound by all of the terms and conditions of the Registration Rights Agreement
and this Amendment, and the Company and the other Investors hereby consent to
the addition of such party as an Investor.

           (b) PEP III and PEOP III and each Carlyle entity other than Carlyle
Cayman are hereby deleted as parties to the Registration Rights Agreement and
shall have no

<PAGE>


further rights or obligations under the Registration Rights Agreement and
accordingly the Company shall have no further liabilities or obligations to PEP
III or PEOP III or any of the Carlyle entities other than Carlyle Cayman and
none of PEP III, PEOP III or any of the Carlyle entities other than Carlyle
Cayman shall have any further liabilities to the Company in respect thereof.

           (c) "Providence" as used in the Registration Rights Agreement and
this Amendment shall refer to Providence Equity Hubco (Cayman) L.P.

           (d) "Carlyle" as used in the Registration Rights Agreement and this
Amendment shall refer to Carlyle Hubco International Partners, L.P.

           2. AMENDMENT TO SECTION 1(a). Section 1(a) is hereby amended to
delete the references to "Investors" in the last sentence thereof and to replace
such references with "holders of Registrable Securities."

           3. AMENDMENT TO SECTION 1(d). Section 1(d) is hereby amended to
delete all references to "Investors" and to replace such references with
"holders of Registrable Securities."

           4. AMENDMENT TO SECTION 2(a). Section 2(a) is hereby amended (i) to
delete the phrase "including any proposed registration of the Company's Equity
Securities by any third party" and to replace such phrase with "other than any
proposed registration requested pursuant to Section 1 hereof," (ii) to delete
all references to "Investor" and replace such references with "holder of
Registrable Securities" and (iii) to delete all references to "Investors" and
replace such references with "holders of Registrable Securities."

           5. AMENDMENT TO SECTIONS 2(b) AND 2(c). Sections 2(b) and 2(c) are
hereby amended (i) to delete all references to "Investor" and to replace such
references with "holder of Registrable Securities" and (ii) to delete all
references to "Investors" and to replace such references with "holders of
Registrable Securities."

           6. AMENDMENT TO SECTION 3(a). Section 3(a) is hereby amended to
delete the reference to "Investor" in the second line thereof and to replace
such reference with "holder of Registrable Securities."

           7. AMENDMENT TO SECTION 4. Section 4 is hereby amended (i) to delete
all references to "Investor" and to replace such references with "holder of
Registrable Securities" and (ii) to delete all references to "Investors" and to
replace such references with "holders of Registrable Securities."

           8. AMENDMENT TO SECTION 5. Section 5 is hereby amended to delete all
references to " "Investors" and to replace such references with "holders of
Registrable Securities."

           9. AMENDMENT TO SECTION 6. Section 6 is hereby amended (i) to delete
all references to "Investor" and to replace such references with "holder of
Registrable


                                       2

<PAGE>

Securities," (ii) to delete all references to "Investors" and to replace such
references with "holders of Registrable Securities" and (iii) to delete all
references to "Investor's" and to replace such references with "holder's."

           10. AMENDMENT TO SECTION 9. Section 9 is hereby amended by adding the
following defined terms:

                "Investor" means each of Carlyle, iaxis, Carrier1 and Providence
         and any transferee thereof who is an Affiliate (as defined in the
         Shareholders Agreement) of the transferor and who has become party to
         this Agreement."

                "Shares" means the Common Shares beneficially owned by the
Investors or any transferees of the Investors who become parties to this
Agreement.

           11. AMENDMENT TO SECTION 10(e). Section 10(e) is hereby amended to
add after the phrase "successors and assigns of the parties hereto" in the first
sentence thereof the phrase "PROVIDED that such transferee acquires Registrable
Securities in accordance with the terms of the Shareholders Agreement."

           12. FULL FORCE AND EFFECT. Except as provided in this Amendment
Number One, the Registration Rights Agreement shall continue in full force and
effect in accordance with the provisions thereof.

           13. GOVERNING LAW. The corporate law of Luxembourg shall govern all
issues and questions concerning the relative rights of the Company and the
Investors. All other issues and questions concerning the construction, validity,
interpretation and enforcement of this Amendment shall be governed by, and
construed in accordance with, the laws of the State of New York, without giving
effect to any choice of law or conflict of law rules or provisions (whether of
the State of New York or any other jurisdiction) that would cause the
application of the laws of any jurisdiction other than the State of New York. In
furtherance of the foregoing, the internal law of the State of New York shall
control the interpretation and construction of this Amendment, even though under
the jurisdiction's choice of law or conflict of law analysis, the substantive
law of some other jurisdiction would ordinarily apply.

           14. COUNTERPARTS. This Amendment Number One may be executed in two or
more counterparts, each counterpart being deemed to constitute one and the same
instrument.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]



                                       3

<PAGE>

                  IN WITNESS WHEREOF, the undersigned have entered into this
Amendment Number One to the Registration Rights Amendment as of the day and
year first written above.

                                    HUBCO S.A.

                                    By:  /s/ Thor Geir Ramleth
                                         -------------------------------
                                         Name:    Thor Geir Ramleth
                                         Title:

                                    PROVIDENCE EQUITY HUBCO (CAYMAN) L.P.

                                    By:  Providence Equity Hubco (Cayman) G.P.
                                         Co., its general partner

                                         By:  /s/ Glenn M. Creamer
                                              --------------------------
                                              Name:    Glenn M. Creamer
                                              Title:   Director


                                      S-1
<PAGE>

                                            CARRIER1 INTERNATIONAL S.A.

                                            By:  /s/ Terje Nordahl
                                                 -------------------------------
                                                 Name:    Terje Nordahl
                                                 Title:   V.P. and C.O.O.


                                            iaxis B.V.

                                            By:  /s/ Abteen Sai
                                                 -------------------------------
                                                 Name:    Abteen Sai
                                                 Title:   Director


                                      S-2