You have to ask what kind of riding your doing. Most of the suggestions I see here are for slower trail riding , slow paced, off camber, mud crawling. If thats your riding style then a
4X4 is the way to go. But if your into what we do here in so cal, which is fast paced trail riding, sand dunes, and wide open riding, sliding turns and jumping then you would want to go with a sport quad.
Everyone wil have their preferences, one guys says Honda is the way to go, another says Kawasaki....I will tell you that INMO, and I am only speaking from sport quad experience, the YZF450 by Yamaha and the Suzuki Quadracer 450R are arguably the 2 best sprot quads available. Those bikes have unbeliveable suspension and reliable, powerful motors.
Good Luck!! Have fun!