The 4 pin is where you would connect your starter button (on two of the pins), one side is hot, the other side to ground. When you press the button, it completes the circuit, and the relay closes, giving full juice through the fat gauge wires from your battery to the starter.
You can see in my photo, the small red lead is from the 12V source and goes to one side of the blade connectors, the black alligator clip is in place of a switch, and I was touching it to ground to roll the starter. On the two lugs of the solenoid, one is battery 12v, the other is the starter cable.