I am a big fan of four wheel drive. Too many times have I seen teams use a two wheel drive with casters, and not be able to control it. Either they turn about too spontaneously, or they can't focus enough power in one direction to get up a ramp... ick
Four wheel drive really isn't as hard as it seems. The key is to make sure that your wheels aren't super, super sticky because you need them to slip enough to turn. With four wheels, you still get a zero turn radius... but you also get pushing power
