Class-4 telephone switch
（重定向自Tandem office）
A class-4, or tandem, telephone switch is a U.S. telephone company central office telephone exchange used to interconnect local exchange carrier offices for long distance communications in the public switched telephone network.
A class-4 switch doesn't connect directly to any telephones; instead, it connects to other class-4 switches and to class-5 telephone switches. The telephones of service subscribers are wired to class-5 switches. When a call is placed to a telephone that is not on the same class-5 switch as the subscriber, the call may be routed through one or more class-4 switches to reach its destination.