There are a few guys who still ride 2-strokes in MX, but Id say the majority of bikes are 4-strokes.
The powerband is so much wider on a 4-stroke, so it gives you a much wider margin of error in terms of timing your shifts.
Also, 4-strokes have a lot more engine braking, so you can run a lot deeper into a corner than on a 2-stroke. When they first switched Moto GP over to 4-strokes a few years ago, the riders found out that they could actually pull FASTER lap times even though it didnt feel like they were riding any harder.
Sorry to get off-topic, just answering 2Blue's question.
Cap'n