I have always mounted my photographs on 4 ply with a 4 ply window hinged to it. Occasionaly, if the corner gets dinged on the 4 ply mount and it's ruined unless you cut it down. Recently I have begun mounting my photos on 2 ply and attaching that to a 2 ply backer of a larger size with clear photo corners, with a 4 ply window hinged to it. The whole package is now still only 8 ply thick, but since the photograph and its mount is sandwiched inside, the corners will not get damaged as easily.
Tim