Dont understand 1/4" deep gap?, but, as Minnflyer said, fill it with epoxy or glue or thick ca if for nothing else, to get the strength back from whatever caused this "flaw"" in the structure.
Dont try to completely fill the gap with the glue though, if overdone, it will not sand down at the same rate as balsa or plywood and youll be left with a hump with thin balsa on each side.