I an very suspicious of a 150 that can only turn 4200 RPM on a 20 foot hull.
Sometimes the cables hang up and the last couple of inches of travel on the control handle is very difficult, making you think you have full throttle when you do not. In the driveway, push the control handle to full throttle. Then go back and remove the hood. Check that the throttle stop is against the block. Next remove the carb covers and be certain the carbs are opening fully. Double check timing to be certain that it is at the requires 30 degrees before top dead center at wide open throttle.
Go back to the force forum and watch the video: Tutorial about synchronizing timing and carbs.
THEN, if everything is correct consider dropping several pitches. At 4200 RPM, if you are using a 19 pitch prop you would need to drop 4 pitches to a 13 X 15 prop giving you about 5,000 RPM. However not knowing what pitch prop you have currently it is not possible to recommend dropping pitch yet.
Upon further thought: Are you using a thru-hub exhaust prop? That engine is a dual exhaust model unless someone has changed the lower unit, Using a closed hub prop will lessen performance.