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?
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.
Or else, use 2WD with 2 extra "large size" idler wheels [much larger than casters] on the opposite end from the drives. This guy has some ideas,
http://www.amazon.com/Build-All-Terrain-Robot-Robotics-ebook/dp/B001UQ5HW4
For indoor use on smooth surfaces, a caster system will work, You might check this out,