The volume of each layer will be the area of each layer times the thickness of each layer.
If each layer is a square, for example, the side of the square will be 7 (28/4) and the area of that square will be 7 * 7 = 49.
If the thickness of each layer is 1 unit, the volume of each layer will be 49 and the volume of 6 layers will be 294 cubic units (49*6).
That answers the question, but it is certainly possible that other shapes will work too..