Engine most commonly used around the world. although on a dirt bike they perform terrible because compared to a 2 stroke, they put out more torque rather than HP and torque is not really needed on a 150 lb machine to get up to speed.
Four stroke rider- hey wanna race
Two stroke rider- sure
Four stroke rider- how come you won, my top speed is higher than yours
Two stroke rider- because I got to a
higher speed faster than you could