I just finished a Disappearing 9-Patch using all 5 inch squares. It is 13 inches finished.
Four inches would be too small, but starting with 4 1/2 inch squares would make 12 inch finished 9-Patch before you cut it up to disappear it. Cutting it takes up another half inch, so you would end up with 11 1/2 finished.
4 3/4 in to start would come out to 12.5 finished.