Building supply agencies sell sand in "yards" by which they mean cubic yards. Thus you need to calculate the volume using yards. There are 3 feet in a yard and 12 inches in a foot so your dimensions, in yards are:
30 ft ( 1y/3f ) = 10 yards on each side. The depth is 4 inches ( 1f/12in ) ( 1y/3f ) = 1/9 yard deep.
Thus the volume is: 10 10 1/9 11.1 cubic yards.
So you should probably order about 11 "yards" of sand.
[Thank you to our alert reader Kevin, who identified an error in the original version of this quandary!]