The 4 contacts are from 2 additional contacts, call them A & B.
With the main switch on, contact A is open and B is closed. Turn the switch off and A closes and B opens.
You wire your ignition feed through B so it kills the engine.
Contact A is used to ground the electrics via a large resistor which should have been supplied with the switch. This gives the output of the alternator a route to earth when you disconnect the battery with the engine running, or you can damage it.

But these small auxiliary contacts frequently fail!