An 18 pitch prop would be about right for your boat and engine combo. Even if it was repitched, don't they only change it by up to 2" on stainless or so? I do not think the root of the issue is a prop problem. 16 to 20 pitch will be ballpark the correct pitch. 90 PSI, if your pressure gauge is accurate, is on the low end, but you should still be able to get to at least 4500 RPMs since they are all even. Closer to, or past 5000 RPMs with the correct prop.
1. Can you get the RPMs past 3800 at idle? See how high you can get the RPMs by using the idle accelerator lever on the remote. If you can redline the engine with the idle accelerator lever on the remote, then your linkage is probably good.
2. If it is still not redlining at idle with the idle accelerator lever on the remote, take the throttle cable off the accelerator lever on the engine (leave the engine in neutral). Move the accelerator lever all the way with the engine running. Does the engine redline now with you manually moving the accelerator lever? If it does redline, but did not for the first test, then your linkage needs adjusting.
If you answer those two questions, I can follow up with further advice from there.