This photo shows the internal wiring of an OMC outboard starter solenoid. You can see the two tiny wires coming from the small terminals on the solenoid and they go into the magnet coils windings, which is encased in epoxy here. The magnet coil is hooked in series and acts as a resistor, preventing full battery power from reaching the mercury switches.
A four terminal Ford style solenoid looks similar from the outside but is wired differently inside. As Frank stated, it will allow full battery power to reach the mercury switches, which isn’t good.
