There have been many threads on this subject...
I totally disagree with the above statement! (except for the fact that they cost more!)
2-stokes don't foul the plugs (if you mix your gas right), they last just as long (if you take proper care of them), they are just as quiet (they both have the exact same mufflers!!), there's zero smoke if you use synthetic oil mixed @ 100:1, they cost less, look better & fit in to more frames, they have far less parts to break, they are much faster!! (there's never been a 4-smoker that could keep up with us on our rides!!), more high performance parts for them & they get 90MPG even after totally hot rodding them!!!
4-strokes are heavier, co$t three times more, they are slower & have way less hill climbing power, they have ten times more parts that can break, require super wide pedal cranks to clear the engine, they have centrifugal clutches (that blow), they're ugly, (they look like you put a lawn mower engine on your bike), they only have pull starts (which break), & no one makes high performance parts for them!