What about making the black border a little larger on the top and bottom than it is on the sides, does the black need to be 4"? Could it be 4" on the sides and 7" on the top and bottom, perhaps with some simple bias applique or fun quilting to fill the space?
Alternatively, could the piano keys on top and bottom be a thin 2" row, then a 1" black strip, then the 7" that are on the sides?