RE: 4 stroke, for speed planes?
Fourstroke on speed planes?
First get rpm up to above 20,000! To do that, you need different camshafts and square props. Nobody but you will do that for you.
So for speed, stick with two stroke engines that can be made to 30,000 rpm and up. For the layman, 25,000 rpm can be bought off the shelf.
Example:
A prop 10x14 narrow blade will need between 5 and 6 hp to run at 20,000 rpm for a speed of about 400 km/h IF the plane will allow that, so it will need a very small frontal area with extremely low resistance.
Doable with a 4-stroke? certainly, if you know what to do, and how to do it, and have the funds to do it.