Hmmmmmm... I had thought that r6rs compatibility mode wasn't very useful, if it just made car a synonym for mcar etc. But it does more, for example unlike in regular plt-4 where in (lambda args ...) args is an immutable list, in r6rs mode:
(import (rnrs) (rnrs mutable-pairs (6)))
(define x ((lambda a a) 'a 'b 'c))
(set-car! (cdr x) 'd)
$ plt-4.1.5/bin/mzscheme -t a.scm
(a d c)
This looks like it could solve a lot of problems with a port to plt-4, since otherwise we'd need to be rewriting the Arc compiler to change the expansion of (fn args ...) etc.