Shay did you considered using 8 wheels? we did last year with the same 4" wheels and it worked really good. you can also look at 2010 robot and see the same idea of 8 wheels the different is the size.
the main though was to do the 2 middle wheels in each side a little bit lower. this gives you easier turns & in case of big bumpers (2010) with the right size of wheels its will be helpful. the problem of making this idea with 3 wheels is the robot is unstable.. can I ask why did you go with 6 wheels & 2 gearbox?
and I guess that the 4" comes to give a lower gravity center?
to see the robots I talked about you can go on our site under History and choose the year
emekhefer2630.com