1 power source 4 motors the motor that loses traction will spin and the others will stall out.
I have ran Izzys loader as well as many others with no problems and yes we do play with it in his yard which is a loamy sand a very heavy combination.
Maybe you should scratch build one and show us how its done.
http://www.youtube.com/watch?v=rrPnFYrQjo0
http://www.youtube.com/watch?v=h0Yt6K45YNE
http://www.youtube.com/watch?v=AfYlIcURP0U
http://www.youtube.com/watch?v=ETjkHdpHFY8
Travis