The first thing you should think about with 4 whl drive is that the robot will turn with difficulty. Have you tried it on carpet with the kitbot parts? Not the same as driving on a slick tiled floor or hardwood gym floor.
In addition, powering drive wheels independently works until the robot lifts in a pushing match. Then less power is transferred to the carpet. You will probably be better off (more competitive) if you use dual CIMS to each side. Do a search for 6 whl drive on this site. There's tons of info on this topic.
In addition, AndyMark and Banebots have dual CIM transmissions available, saving weight from a four transmission setup.
