I know for me, when I am in my best shape, my step has to be 10" high or my heart rate won't even hit my training zone. Obviously, the women in Cathe's videos are in great cardiac shape so hence, the 5 risers.
But fitness is personalized so I wouldn't feel pressure by what others are using. Just use what works best for you and what your goals are. If you are getting a great workout with 4 risers - then that is what works for you.