Understand a smoother response from 4 subs before cal puts less stress on the filtering algorithm,
No that's missing the point. Equalizing or getting flat response is only part of the problem. Another equally important factor is smooth bass is fast articulate bass.
That's because bass energy is in the room whether its measured flat at your EQ location or anywhere else. To get flat bass with EQ always requires turning it up somewhere. Even if its not EQ'd louder still there's extra bass energy somewhere in the room for the simple fact that's the way bass works- there's always reinforcement somewhere, cancellation somewhere else. Nulls and sickouts aren't a problem. Reinforcement is, because that extra bass energy hangs around taking time to dissipate which until it does results in muddy bass. This probably more than anything else is why DBA bass is so exceptionally fast and articulate.
More subs is diminishing returns only because once you have reduced one set of really big modes to two smaller ones they're half as big. Then form 2 to 4 half as big again. To cut them in half again means doubling to 8 which as Duke says is grounds for divorce. But I went from 4 to 5 and it sure did not seem like diminishing returns to me. In any case the returns are relative to DBA not EQ, which is apples and oranges.