The Tektronix 4S2 is a two-channel sampling vertical plug-in for the 661. Signals enter the 4S2 through GR-874 connectors on the front panel. Unlike the 4S1, the 4S2's signal path does not contain a delay line between the input connector and the sampler. This has the advantage of maintaining higher pulse fidelity and thus higher bandwidth. It has the disadvantage of making it impossible to see the leading edge of the triggering event unless a pre-trigger is used.
The 4S2 uses a four-diode sampling bridge. The 4S2A uses a two-diode sampling bridge. This design change is discussed in the 4S2A Engineering Spec document linked below.