As is alluded to, the bumpers do not need 4" high numbers on them. It is a convenient place to satisfy the rule that you must have numbers on all four sides that are 4" tall, but they can be anywhere as long as they are clearly visible. If the bumpers did need the numbers, then it would be impossible to satisfy that rule while taking advantage of only needing to use bumpers on 2/3 of the robot, and I would be very unhappy
