X v3 and X v4 can coexist. The main package you're looking for is xserver-xfree86. Start from there.

You'll need a new config file. If you really want to, use xf86config to generate it (the old version should have been renamed as xf86config-v3).

/etc/X11/X should be a symlink to /usr/X11R6/bin/XFree86
/usr/bin/X11/X (aka /usr/X11R6/bin/X) should be a small (7k) suid root wrapper.