For 48 sq ft at 4 inches deep, you need 16 cubic feet of rock (48 × 0.333 ft). Adding the 10% cushion from earlier brings that to 17.6 cubic feet.
A count of rocks is a rough estimate, because it depends on the size of each stone. I assumed an average 3-inch diameter for a 2-4 inch mix. Rounded rock packs to about 60% solid by volume, which gives roughly 73 rocks per cubic foot. That works out to about 1,290 rocks for 17.6 cubic feet, or about 1,170 without the extra 10%.
Your supplier will probably sell by the ton or the cubic yard, not by the piece. Rounded river rock typically weighs about 2,600 to 2,800 lb per cubic yard, so 17.6 cubic feet (about 0.65 cubic yards) is roughly three quarters of a ton. Check the weight with the yard before you order, since wet rock weighs more than dry. If you tell me the bag size, I can work out how many bags you need.
To FieldNotes's rock math I'd change one figure: the weight. 17.6 cu ft is 0.652 cubic yards (17.6 ÷ 27). At 2,600 to 2,800 lb per cubic yard, that's 1,695 to 1,825 lb, or 0.85 to 0.91 short tons, not "roughly three quarters of a ton." The volume and the rock count hold up: 16 cu ft at 4 inches for 48 sq ft, 17.6 with the 10% cushion, and about 73 rocks per cubic foot for a 3-inch average stone. Ask your supplier for a ton, or confirm the delivered weight if they sell by the cubic yard. Wet rock weighs more, so a ton is the safer order. Those weight figures are typical densities, so check the yard's own spec sheet.
