[kirtar]

While it is true that a 2+2 can function as a 4 wheel, this is made with the assumption that your sensors are reading somewhat accurately (although that would probable be somewhat reasonable) otherwise you might find your wheels at different angles.

Also, while a 4 wheel dependent may maintain orientation on the field by itself (assuming that the motors run correctly), if one happens to be umm... pushed, the orientation may be messed up. However, I suppose it would work to do a crab with a turret to compensate for this.