If you can get 4inch Omni wheels, I'd think those would be a better option. 111 had a long base robot back in 2006 and we used 2 omni wheels and 2 traction wheels. If you zoom in on this picture, you should be able to see them.
http://www.wildstang.org/main/robotpix/robot2006.jpg
I wish I could post a better picture, but our photo album doesn't seem to want to work for me.