The first thing you need to know is "what pump" you have? The specs will let you know pressure and volume.
1 psi is not enough for good performance but it will run at low engine speed and load. 2~3 psi is very good.
I have not seen a return line on an A-4 but heard of a couple who installed one, One was for vapor lock however I don't see how a marine engine could need one unless it was really overheating and some use it a polishing mechanism.
As far as the OPSS mount it where it is the most convenient for your installation regarding future maintenance.
If you are in doubt about your pump best to get one of the proper rating from our host as it will "be correct".
If the current pump was not wired how did it work or was it just mounted as a spare incase the stock pump failed?
Dave Neptune