Having a global "R4RS mode" won't really work, because most code is
written to expect either one behavior or the other. Having a
per-compilation unit mode of some sort wouldn't work, because you
really want to be able to pass data between compilation units, and
that data might contain #f or nil values.
We're just planning on keeping #f and '() equivalent.
I know this is a pain. I've grown to like the #f/() split myself.
But Emacs Lisp compatibility is very important, and we haven't been
able to come up with a cure that wouldn't be worse than the disease.