If the prop is too big it will never let the engine spin up to where it makes it’s real power in the upper band of revs. Rough example of you may have 3/4 of throttle but only half of the power. Engines need to be free to eat!

Prop pitch is 21...I will try a 17 or 18 tomorrow and see if I can get my 4500RPM

I know, oldish thread, but it's interesting.
It does need the smaller pitch propeller due to the different drive ratio.
But as said, I'm interested. It might probably still need that wiring issue addressed to get wide open throttle to run past 3800 rpm.