Notice that 4 inches is 1/3 of a foot.
So the volume of that space is (24 x 24 x 1/3) = 192 cubic feet.
If you buy it in sacks, the sacks are labeled in cubic feet, so 192 is
the number you need. But that's an awfully expensive way to buy it.
Better to have it delivered all in one truckload. When you order it
that way, they want to know how many cubic yards you want,
so you need to know that 27 cubic feet make 1 cubic yard.
192 cubic feet = 192/27 = 7.111 cubic yards.
Buy 7 cubic yards on a truck, and the leftover 3 cubic feet in sacks.