XFree86 4.0 is the first official release of the new XFree86 4.
XFree86 4 represents a significant redesign of the XFree86 X
server. It is very important to keep in mind that XFree86 4 is
still very much in development, and it contains a lot of new work.
That means two things: there is a lot of new exciting stuff to try,
but being new code, it hasn't had nearly as much of a workout as
the stable 3.3.x releases. If you're looking for a well-tested,
stable release, and can't afford the inconveniences that new
software can sometimes cause, then you are probably better off
sticking with the 3.3.x releases for now.