EXHIBIT 4
JOINT FILING AGREEMENT
In accordance with Rule 13d-1(k) promulgated under the Securities
Exchange Act of 1934, the undersigned agree to the joint filing of a Statement
on Schedule 13D (including any and all amendments thereto) with respect to the
shares of Concentric Network Corporation and further agree to the filing of this
agreement as an Exhibit thereto. In addition, each party to this Agreement
expressly authorizes each other party to this Agreement to file on its behalf
any and all amendments to such Statement on Schedule 13D.
Date: October 28, 1998
SBC COMMUNICATIONS INC.
By: /s/ Xxxxxx X. Xxxxxxx
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Xxxxxx X. Xxxxxxx
Senior Vice President,
Treasurer and Chief
Financial Officer
SOUTHWESTERN XXXX
INTERNET SERVICES, INC.
By: /s/ Xxxxxxx X. Xxxxxxxxxx
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Xxxxxxx X. Xxxxxxxxxx
Secretary
