How fast is a 4 HP boat motor?

It depends a lot on the size of the boat. If it say, a 12 foot boat, probably about 7 or 8 knots (about 10 mph). On a bigger boat slower. As was said these small engines are used mainly for dinghies and for fishing.

Answer: I have a 8 foot fiberglass dinghy with a 4 Hp outboard. My GPS says 11km/h under full throttle on a calm lake.