I've seen several 4500s with soft floors in the area near that seam in the floor. Bad design. If I was working on a 4500 that did not have soft spots, I'd be tempted to fiberglass that seam. I've also seen a few with rot in the rear dinette area. Some have pieced in plywood to repair the floor on the rear dinette.
I've posted a picture on the Trillium FG where someone removed the body from the frame, rolled it on its side, cut out the bottom of the trailer, replaced the plywood, repaired the fiberglass, and put it back on the frame. That qualifies as a MAJOR job.
Plywood is exposed on my 1300 at the edge of the front gaucho (inside the bench) and underneath the
refrigerator. The pontoon design offers some protection in those areas.