The truss rod system on the 4001 is archaic - you're supposed to manually set the neck into the position you want and you use the truss rods to lock it into place. The 4003 has a redesigned truss rod system which works in the contemporary way you'd expect. As mentioned, the neck of the 4001 wasn't designed to handle roundwounds.
I used to own a 1991 4003 and, above everything, it was the tone I loved - aggressive, growly and fat, almost too punchy, a wonderful experience. I had a guy with a 4001 in the studio a few years back and it was heartbreaking- thin, weedy, lifeless tone out of the thing was such a disappointment.