4 strokes are not going to give you as much top end speed as a two stroke of the same horsepower. It is just because it is a 4 stroke. With a 4 stroke you only get power to the prop every other stroke. With a 2 stroke you get power to the prop every stroke (I hope I explained that right, if not someone with more knowledge of motors help me out). So 2 strokes get higher rpms. 4 strokes are also heavier. Short of trying out different props there isn't much you can do. That is just one of the downsides of 4 strokes.