A 4 stroke goes around twice for every power stroke, a 2 stroke produces power with each revolution so it makes less power. They also (in general) run lower rpm and more torque which makes them better swinging larger propellers slowly where 2 strokes come on in the higher rpm bands swinging smaller props.

4 strokes also sound better.

Here's an article http://www.masportaviator.com/ah.asp?CatID=8&ID=95