Yamaha is insane. Four-stroke technology is not suitable for snowmobiles at this point. Try to apply it, and you end up with an underpowered, overweight machine-or a Yamaha Nytro.
Yamaha claims it can keep up with the best two-strokes. Let's test that, using the Nytro against a Bombardier machine. The Nytro weighs 650lbs+. The Bombardier weighs 440lbs or so. The Nytro has a 1000cc triple cylinder four stroke motor, rated at 130hp. The Bombardier is powered by an 800cc two stroke, rated at 151hp. Speaking from experience and feedback from others, the Bombardier also handles much better. See a pattern here?
AND, after all that, the Yamaha is also more expensive, and not any cleaner burning.