I have a 4 foot 120. I have 2 WP mounted on opposing sides. BUT I use each one at a time to simulate tidal flow. I get plenty on waves using the off on speed selection. I also run them at about 16 volts not full speed. Two WP are too much for a 4 ft tank if you plan to run them at the same time or if you try an anti - synch mode. I can can easily get a wave structure that will over lap the top of the tank...and I have a lot of rock in this tank. I suggest you try one and evaluate you wave action. Every tank is different but that's my experience.
RJ