If you mean 4WD with the drive wheels close together, and 2 casters on the ends, that sounds doable, but I don't see a situation where that design will be at all useful. IE, why even bother with 4WD?
Yes that is what I mean. The only reason I'm doing it that way is that I have 4 of the same motors. If I understand torque correctly using 4 drive wheels will double the torque the robot would have if It was using only 2 drive wheels. To push the robot up the needed incline (I don't remember what it was off the top of my head) two of the motors I have will not have enough torque. That is why I need four drive wheels.
I think a lot depends upon where this robot will operate most of the time. If outdoors on earthen terrain, I don't think having casters is a good idea at all, as they will probably dig into the dirt a lot. For that, I would go with a basic design with 4 drive wheels on the corners of the robot, and relie on a very robust motor/wheel system to keep it from breaking down.
The caster wheels will be fine (link below). Also, I was going to put the drive wheels towards the center to improve the turning. I don't believe the motors will break. I need the turns to be exact when its driving autonomously and I think this design will give me the needed torque and turning capability.http://www.trossenrobotics.com/store/p/5881-Parallax-Caster-Wheel-Kit.aspx