Feet and square feet, or yards and square yards, are not comparable because they have different dimensions. Feet and yards measure length (single dimension) and square feet and square yards measure area (two dimensions).
But let's look at your question. 4 inches is a third of a foot (4 inches is a third of 12 inches) and the area in square feet is 140/3=46 and 2/3 sq ft or 46.67 sq ft.
What depth is the ledge? Concrete will be sold by volume or weight, not by the yard, which is a linear measure. When mixed it is used to fill a volume. If concrete was sold in lengths it would be lengths of a particular width and a particular depth, which together give a volume=length*width*depth. Paving slabs, for example, are sold by area and thickness, and area*thickness=volume.