One point appears to have been missed on HP rating and that is your engine mount. Most have weight and HP ratings. Some of the older ones were designed for 2 stroke engines and not 4 stroke. 4 stoke has more thrust initially and will bend some of the old mounts at the same HP rating. Also hanging too much weight on the stern of a boat is a good way to slow it down.
If you use Dave Gerr's formula for hull speed prediction, you may go a little faster with the a slightly larger size outboard than the average 4HP on a 20' boat.
If you use Dave Gerr's formula for hull speed prediction, you may go a little faster with the a slightly larger size outboard than the average 4HP on a 20' boat.