A photo that large will be difficult to keep flat
under any circumstances, but with corner supports
and a 4ply mat, they are probably being pressed
between the glazing and the backing, since the
window is providing only 1/16" of clearance. The
first question to address is whether the photos
are flat, when they come in. If they are laid on
a table and are not completely flat, they will not
be flat when they are vertical in the frame.
Hanging them from hinges will, as noted, allow
gravity to pull the weight of the photo down and
that may help. It is certainly a better approach
than corner supports, which will be placing the
weight of the photo on the weakest part of the
sheet, its corners. This is a classic example of
why corner supports should not be preferred: they
do not support the center portions of the edges of
the sheet and that is where support is needed.
If there is an ample white margin on the edges of
the photo, folded paper edge strips, with ample
overmatting might hold the photo flatter, but
hinging may be the best option, here.
Hugh