4 stroke engines GENERALLY are not used on smaller RC cars, ie 1/10, 1/8.
The reason is because they are heavy when compared to a 2 stroke that produces the same torque at a given RPM. 4 strokes don't produce their torque at as high of an RPM that the 2 strokes can. A 2 stroke engine when compared to a 4 stroke of the same displacement can produce WAY more RPM. Weight is everything in these cars. You are fighting un uphill losing battle against the laws of physics if you think your going to find a 4 stroke that will compete against a high end .12 .21 or .28 two stroke RC engine...
Four strokes are generally only use in larger 1/5 scale or 1/4 scale cars...
Now that being said. If you were to come up with a forced induction 4 stroke RC engine then you might be able to get your power/weight up closer to that of a comparable 2 stroke rc car. Keep in mind you would need to gear the heck out of it. Since the 4 stroke would be producing all its power down low on the RPM range and its max RPM still would not compare to a 2 stroke.
RC's: MUGEN MBX5-T Prospec w/Ninja .28!
MUGEN MRX4X w/ Novarossi 321P Tuned
Trex 450 V2
Real Car: 2003 Corvette C5 Z06 Millenium yellow