They would only need a 2.7 gallon (USG) bucket to bring up 4 tons per day.

But please check my estimate: A cubic yard of sand can weigh approx 3000 lbs, so that's 2/3 of a cubic yard per ton. 27 cubic feet per cubic yard, so 18 per ton, which is 135 USG. 4 tons (from your question) is then 540 gallons, and at 200 dives per day that is 2.7 gallons per dive.

I'd bet they do more like 8 tons per day per diver.