4 inches is plenty especially if you use fine sand. I have a lot of current in the tank so I put a layer of coarse shells on top of that to keep the current from blowing the sand around. I have had 10-12 inches fine sand before and did not see much different in term of ability to process waste.
My nitrates always is 0 with heavy fish feeding and heavy fish load. I change water for trace elements and micro nutrients not due to Nitrates