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.
200 dives a day, lets say they are super and work a 10 hour day (for easy math).
That's 20 dives an hour or one every three minutes.
Diving down 40 feet and bringing up a 3 gallon bucket of sand, once every three minutes for 10 hours straight again seems... very difficult (or maybe a bit improbable).
Perhaps it's a special kind of sand and they get more money for it so they don't have to do quite that much?
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.