My neighbors fence has gates on that scale, mounted to 4 X 4 posts, and
they seem to work OK although they are seldom opened. Your plan sounds
OK to me. Don't skimp on the hinges; use real sturdy ones and mount
with lag bolts. The hinges will take up some space - instead of just
one cut to halve the fence section, you may have to make two cuts to
remove an inch or so of it (i.e. make each half about a half inch less
wide). Zero sag may be hard to achieve even with diagonal bracing; you
may want to initially mount them pointed up slightly to compensate. --
H