they would certainly work with a four stroke... as long as they are well made. in other words, the fan blades are well machined and dont loose a lot of air from around the duct housing and blades themselves. another difficulty would be that the carb is behind the case on a four stroke.... so how exactly would you get the thing spinning in the first place since the crankshaft is on the other side of the motor? the best thing to do is literally custom build one for a four stroke. i once read an article about boosting an OS .91 four stroke, it increased the HP and torque about 20-25%. but thats only because they did a VERY good job at it and built it to very tight tolerances.