4,000 RPM

I think you want to be more like 4600 to 4700 rpm talk to brett he said you could try a REV 4 MERC prop or if that don't work a mirage but 4000rpm wfo is to much load on your motor.

Like was said that is very low RPM. Find out what is your your speed on GPS and the RPM at WOT. Your tach may be off, some of the tachs have a switch on the back for 4,6,8. How many hours are on the engine, maybe you need a tune up? If everything is working correctly you may need a 17 pitch.

This was in slight chop w 4 people on board.
4,000rpms, 44mph. 180 hrs on the motor, Assuming the tach is correct then I was at 4,000.

I have an 1986 Scorpion with the 260HP 5.7 and an Alpha Lower, GPS shows 42 mph with tach at 5200RPMs with an aluminum Mercury 14.5 x 19 pitch Prop. Mine will shoot out of the hole with 6 people in it and a skiier on back. It's odd that the two would be so different. I'm looking into trying a 21 or 23 pitch prop to lower the RPMs back down a little.