One thing to think about is the heat generated from multiple powerheads. It will add up in the end. A 75 is big, but not that big. Those things do put off heat. That said, water flow is the key, and random is better. I strongly recommend the wavemaker powerstips for this size tank. They are not expensive at all, but they only do 3 of the 6 outlets on the timer.
I run 3 maxi-jet 1200 on the wavemaker powerstrip on my 100 gallon, with one more lower-flow PH way down in the tank, in the back of my LR
that is always on. So, I have four like you are talking about, but they are not allways on at the same time - usually only 2 at a time, and they are almost all the 1200s. I have NO dead spots, and the water is very brisk and random, thoughout the tank. I would look into using the more powerful PH and the timer powerstrip, I love the results I got with it.