I think it would be easier to get yourself an aftermarket fuel pressure regualtor with a guage already attached, and also get a greddy fuel rail adapter for a SR20, get like a megan racing regulator, I have one of those, and it has a built in fuel guage.
To run 4bar, you will need your ecu programmed for it, talk to Calum or erik212
P.S. a megan racing regulator will run you about $70 and the greddy fuel rail adapter is about $45, that's what you will pay for a nismo regulator with no fuel guage