The reason 4.x was not finished was
major scalability problems. It would take 4 hours to return a page, sometimes all day.
That may have been fixed with the permission_p fix that has been made, but I am not sure. I just remember discussing data models and etc when the 4.x version was planned.
Also, you need several other packages ported to have it work, most notably acs-calendar, acs-event and (maybe) acs-reference.
