Depends on the size of 4 cycle.... KT-100 with pipe and methanol around 20hp. Clone - 14hp? Flattie -12hp? The biggest difference will be the powerband --- nothing nothing WHAM!! A clutch will soften that somewhat depending on when it's set to engage.
The old open FH's and the KT100's were very close in performance. The newer OHV engines would have more. If you are planning on running the UAS class you need to look at the rules and build an engine to your weight.