4 hp is fine
When I purchased my used C22, It han old Johnson 4hp longshaft on it, I bought the boat at Lake Erie, and used it for a few months there before moving it to Lake Pymatuning. When I sold my 16 ft Neptune, I switched the 4hp to that and kept my Johnson 6hp longshaft. I never thought the 4hp was a problem and have seen them on other C22s. I admit that I never was in bad weather or heavy winds trying to motor with the 4hp. If you do buy a new motor, make sure that you get alongshaft model, to get the propeller down in the water, where it's more efficient and less likely to come out of the water when the boat heels. As far as the weight issue, a smaller motor like you're suggesting would be less weight than the max hp Catalina Yachts specs, so it should be no problem. In fact less weight in the stern means that your boat should point higher when beating. Go to section 4.5 of the attachment to ease your concerns.