Hey RC, good to see that you are still in the game. I experimented with 4000psi a couple of time with the new valve and got in the 325fpe range. If the gun can be shot at 4500lbs the fpe could go up to 350 or so if the rest of the gun can stand the pressure. If the rest of the gun won't stand that pressure, it will give lots of overhead for a regulator. Regards, Tom