The starter relay likely has 2 smaller terminals for signal wires. One of them gets a yellow wire from ignition switch. The other gets ground. If its automatic trans, that ground comes from the neutral safety switch in park or neutral. That can be a brown wire, a brown with yellow tracer? Yellow with brown tracer? Depends on year model.
If its a manual trans, the OEM relay didn't have a 4th terminal. The relay for automatic can and has be used, just jumper the 4th terminal to ground at the relay mounting bolt.