Can the G6B-4-C Terminal Cover be placed on the G3S4 Terminal Solid-state Relay?

No, it cannot be placed on the G3S4-A/-D, but it can be placed on the G3S4-A1/-D1.
The G3S4-A/-D Terminal Solid-state Relay has a heat sink and the Terminal Cover and the heat sink are attached by hooking them on a claw on the case. There is only one hook, so both devices cannot be attached at the same time.

Applicable models: G3S4-A, G3S4-A1, G3S4-D, and G3S4-D1 Terminal Solid-state Relay