"The lower corners are soft where there is the greates distortion caused by the short lens."
I've seen this effect with very wide lenses that will just cover when movements are used. It looks like an increase in coma at the corners of the frame. I'm not sure whether this is actually coma or just an effect of the normal wideangle distortion (or perhaps coma is this effect). Maybe the optical gurus have an idea about this.
Scratch my earlier theory. I was thinking this was like a still life, not a landscape.
If "the softness is in the negative" as you say, then it's not the enlarging lens, although that is a short lens for 4x5".
Another possibility is a film flatness problem in the camera--either the film popping in the heat or sagging with the camera pointed down, if it was pointed down.