Well 2wd might give you more speed you will probably have less torque.
And the idea about have the other 2 wheels free wheeling is useless because if i am right on what your thinking is just having all 4 drill in the robot and then just undoing the bolts on 2 and having the wheels spin freely . That is just wasted space and weight if you have the drill in you may aswell use them. If not just take out the 2 sets of drill you arent going to use and just put casters on the undersided