4 wheels is good with a taller robot because it can lean further to any side before falling over, as compared to a 3 wheel base.
4 wheels is harder to steer because you must build the mechanism to turn 2 wheels at the same time, as well as to prevent them pointing in different directions.
4 wheel robots are more balanced and less chance of going unstable......while 4 wheel robots are usually slow and they r costlier than two wheeled robots