I'm guessing that you have a fair old turnover rate on your tank. If so, that fast turnover in conjunction with your four returns, and powerheads, is obviously causing this problem. You've tried several different positional combos already, to no avail.
Would it be such a problem to dial your turnover down, do your fish need high turnover? I think that's where i'd start, dial it back, see how you go, and turn it back up incrementally until you reach a happy medium between an acceptable turnover and minimal movement of sand.
If you reach a point where your sand only remains dead still with a really low and unacceptable turnover then you may have to think about a heavier substrate if you want your current flow.