How fast would a 21 ft bayhen go with a 4 hp engine?

The speed of a 21-foot bay boat with a 4 hp engine is generally quite limited, likely reaching around 5 to 7 knots (approximately 6 to 8 mph) under optimal conditions. Factors such as the boat's weight, design, and water conditions can influence this speed. In practical terms, a 4 hp engine is more suited for low-speed cruising or fishing rather than high-speed travel.