"I've heard that different manufacturers rate their horsepower differently."
"I even heard that a 4hp four stroke might have less power than a 3.5 two stroke."
Depends...In the case of the Tohatsu/Nissan/Mercury the answer would be no as the 4 is a detuned 6. However, a 4 is too small for a 2200 lb boat. You would need a 5 at a minimum and would be well served by getting a 6 pitch prop.
Solution One Maritime