A small clarification for nate and Greddy...
A 2 stroke fires every time the piston reaches top dead centre (or the top of the bore) whereas a 4 stroke fires every second time it reaches tdc. This is because of the different methods of induction and exhaust that the two motor designs use.
I'm not sure why you would want to use 4 stroke in RC, unless you are racing 1/5th or something, as they are heavier and have far less power per gram. Especially when the 2 stroke motors that are run don't really have to have any long term reliability (not like your everyday car etc).
As for motocross however, the 4 strokes have been somewhat of a revolution, high revving, light and great power output with less overall maintenance and greater reliability. Most guys I know would prefer a YZ426 over its 250 2 stroke brother...you could apply this to MotoGP as well. The development to get the 4 stroke to perform to 2 stroke standards hasn't been invested in RC yet IMO.