A relay in most cases and indeed this one needs two lives, one is a permanent live, and the other a switched live, the permanent will be the feed to the pump when tripped, and the other is for the relay itself. The forth wire should be an earth, in the case of the RST I believe this goes to the ECU and is grounded upon recieving ignition pulses, it also primes for five seconds when the ignition is switched on.

You need to make sure the relay is activating first, it should click if so, if it does not then you need to check the switched feed to it, and the earth is good.

As a last result as a test ONLY you can temporarily bridge the relay input and output, I.E the permanent live and the output wire which will go to the pump itself, when you do so the pump should run permanently, if it doesen't then you have either a pump fault or it's associated wiring.

Martin