How to upgrade to XFree86 4 on unstable ?

[Balbir Thomas <thomas.1037@osu.edu>]

Hi,
I just managed to upgrade my potato box to unstable (with kernel 2.4.5) . I 
used the following source list :

deb http://http.us.debian.org/debian unstable main contrib non-free
deb http://non-us.debian.org/debian-non-US unstable/non-US main contrib non-freedeb http://security.debian.org stable/updates main contrib non-free
deb-src http://http.us.debian.org/debian unstable main contrib non-free
deb-src http://non-us.debian.org/debian-non-US unstable/non-US main contrib non-free

I now find that xserver-xfree86 and xfree86-common are upgraded . I even ran
xf86cfg . But when I try to startx it still runs 3.3.6. How do I change to 4 ?
Is it possible to retain 3.3.6 while using 4.

sincerely
b thomas