If you knew the length (L), width (W) and thickness (T), all expressed in yards, then the volume of concrete in the patio would be
L W T cubic yards.
You know the volume is 2 cubic yards and hence
L W T = 2 cubic yards.
The thickness is 4 inches and there are 12 inches in a foot so 4 inches is 4/12= 1/3 of a foot. There are 3 feet in a yard so 1/3 of a foot is 1/3 1/3 = 1/9 of a yard. Hence
L W 1/9 = 2 cubic yards
L W = 18 square yards.