Part of the problem is that you can't really test the output of the prop sitting at the dock. The top-end RPM will be lower than if the boat was in motion IIRC. The torque and performance curves of two-stroke and four-stroke engines is very different and it is hard to compare them directly IMHO. Also, if you're not getting to the manufacturer's recommended RPM range for WOT, how is it that you expect to get the full HP out of the engine. The missing two HP that you're complaining about is probably provided by the 1700 RPM that you're short of the WOT recommended range by.
Oh I so totally agree with you.. That 1700 rpm is exactly where my 2 hp im looking for is. Based on getting the same results on two engines now. I'm of the opionion that the engine is not producing enough power to turn the lowest pitch prop made for it at nominal rpms. This to me means the engine is under powered to do what it is called for from the factory to do.
regardless of all the numbers (rpm/thrust etc) at the end of the day the biggest difference to me is that a 13 year old 4hp 2 stroke engine will push my boat faster than a brand new 6hp 4stroke.