Find a suitable storage location in your workshop, basement or garage. You will need an 8-foot length of wall to store plywood sheets. The plywood should not be stored against an outside wall, to avoid moisture that may seep through the wall.
Cut the 2-by-4 into four 2-foot lengths. Lay the 2-foot boards flat on the floor, their length perpendicular to the wall. Place one board at either end of the 8-foot space. Measure 32 inches in from either end board and place a board at these locations. You should have four 2-foot boards sticking out from the wall; equally spaced to accommodate the length of a plywood sheet. These boards will prevent moisture from the floor seeping into the plywood sheets' edges.
Stack the plywood sheets upright along their long edges against the wall and atop the four 2-foot boards. The sheets should be sandwiched together against the wall as close to vertical as possible without tipping. The finished side of the sheet stacked against the wall should face out. The rest of the sheets should be oriented with the finished sides facing in. This will prevent warping and surface damage, and allow quick access.