I know that this is not my thread, but I am seriously trying to help... When dealing with waveboxes, tank length and obstuctions (rock and coral) are the main factor. 4' tanks don't make as big of waves as 6' tanks (or even 8). I have one in my 4' 120' and it makes a really nice 1.5" wave for my SPS. Not only does the coral do better IME, the sand does not blow around. This also gets behind and in-between the rocks. This unit should be absolutely fine for a 75G tank. If you want to see a wavebox on a 4' tank, then let me know and you can swing by.
I would have seriously bought this already, but I just went through the exercise of acquiring stuff to make my own since they rarely are available used - most people who have them will never let them go. FWIW, it cost more for me to make one than what this one costs.