2000 4hp has a Flat Spot.

[Ltc]

Under load at 3/4 throttle I hit a flat spot. Nothing happens for the last 1/4. Rebuilt the carb. and put in a new plug. Still flat.

[boats.net]

That’s not that unusual for a engine to do that.There's not a big prop selection for a 4HP engine just 6 pitch and 8 pitch. You might try the lower pitch if not on it already.