Won't make any difference unless your map is designed to run one - Revo stage 2 requires one, for example.
Well, I mean it won't make any difference to performance, the ECU will reduce the injector pulse width to reduce the amount of fuel injected (the higher fuel pressure would push more fuel in for a given pulse) but, I think, eventually it will run out of fuel trim adjustment and throw a faut code.
Theoretically, you could mess around (increase) with MAF tube diameter so the ECU thinks there's X g/sec going in but actually there's X+Y, and then use the extra fuel that the 4 bar provides, but the risk of getting it wrong would put me off. Plus, you'd spend that much time logging (and can you interpret the results?) If you were doing this, you can get adjustable FPRs so you could fiddle with that as well.
Save your money and put it towards Stage 2 requirements.
You can get them from Euro car parts.
Edit: Snap, snap, snap. @Jase - a mate had until recently the pre stage 2 4 bar code on a Leon Cupra - it flew