You would not go wrong with either a new O.S. Max .91 FX or a new SuperTigre G-90 engine. Both are excellent engines that give good power and longevity, with proper care.

So-called "four-stroke" propellers are only those props that have diameter/pitch combinations that have proven to be effective with four-stroke enignes in the suggested size range. They usually are a little-higher-pitched for the diameter when compared to two-stroke propellers. If they allow your engine to run well and the airplane to fly the way you desire, they are OK to use.