The only thing that I can think of to make the cart 4 wheel drive, would be somehow convert the horizontal spin of the chain into a vertical belt drive and hook up an old riding mower rear axle to the steering, the pulley in the center would allow the axle to turn freely. However, this seems to be a bit simple of a solution and I think it would end up getting jammed when you go to turn, or the front axle would try to steer you in whatever direction it wanted to go. I think it would be easier to make your kart into a 6 wheeled setup, 4 in the back for power, and 2 in the front for steering.