Make sure you're including muffler weight when comparing engines. The .46 AX is actually around 17 ounces or so with the sport muffler attached. A .52~.56 four stroke will likely be lighter weight than a .46 AX with muffler.

Too many pilots short change the .52~.56 size four strokes. These are very powerful motors that are every bit as hot as a good ball-bearing .46 two stroke.

A .70~.75 four stroke is really more similar to a .51~.56 two stroke with regard to power output. These engines are great for heavier or high performance "50 Size" airframes, but are complete overkill for an airframe like the Kadet LT-40.

There are a hundred great uses for a .52 four stroke after you decide to take it off of your Kadet LT-40. Don't buy into the notion that you have to have the biggest crank case possible to strap onto the nose of your next plane.

Good luck and good shopping!