I'm not very good at the math, but it seems to me that if you have already cut the blocks 5 inches square, the finished block would be 4 1/8 (or I would trim them down to 4"). Then you could cut the focal block 4". That would give you smaller blocks than intended, but you would have rescued the fabric. You can always use borders or sashing to make the quilt to the size you would have wanted it to be.