most of the new 4 strokes I see have way more problems then 2 strokes, but I think it's because people refuse to belive that the modern 4 stroke is a full race motor that recived most of its technology from f1 engines.
if you choose to go with a 4 stroke: make sure to follow the factory mantiance schedual; and you will forget how to ride fast and use a clutch.
I would not consider myself a fast ridder but i have never had a 450 come close to beating my 500 in the sand. the 500 pruduces more power sooner.