RVator, In my opinion you already have the best prop for that engine on that plane. A 12x4 would not be enough load and let the engine over rev. You could always try the 13x4. Experimenting is part of the fun of this hobby. You should have a tach and that engine/prop should have a 2,000/2,100 idle or maybe just a little lower and that should act as a brake and not pull the plane at all. I've flown the LT-40 with a 52 4-stroke and a 12x6, and several other trainer type planes with the Saito 56/12x6 and it is a very good combo. You might check and make sure that your c/g is not too far forward as a plane that is too nose heavy tends to land faster than a plane that is balanced at the back of the c/g range.
FEB