I think that in the near future you will see 4 strokes from the commercial engine arena that are very close to 2 strokes. The EPA and other numerous agencies are constantly increasing the pressure to reduce emissions and also under consumer pressure for less weight and more power. But for now, you certainly have to pick the airframe for the engine. If you put a 4 stroke gasser in an Extra type plane you will hate it, but on a WW1 or Cub type plane they are great.