They are finally starting to get some good 4 stroke technolgy on to the quads now. I've got a Grizzly.
It has a 660cc Liquid Cooled, 5 valve motor. The thing will do about 70 mph, and its a Utility. Its little brother the Raptor, has the same motor, but 2 carbs. Even more performance. They seem to respond pretty good to pipes/jetts too, although not nearly as good as a two stroke.
LS1, What kind of riding do you want to do? The Big Bear would be a fine choice for you, or maybe a Kodiak 400 if you want 4x4. It all depends on the riding style you have. The most important thing to do, is get out and test ride as many quads as you can. Decide what YOU like.
Wobblin, There are a few more 2-strokes than you might think. The Blaster is one, but there is the Banshee, Gas Gas makes a 300cc two stroke, and Bombardier(sp) makes a 2 stroke sport quad. They are however being phazed out.
-Jared