Hi
I have successfully compiled and configured SAMBA4 beta6 on a Raspberry Pi, I now run it as the Primary Domain Controller (PDC) at home 
I compilied it natively and it took like 2 days.
Raspberry Pi as (PDC) at home, $35 PDC in a cardboard box compared with a cheap Windows PDC about $3500.
The procedure is flat out of the HowTo at their wiki except that I had to increase the wheezy cache to 300M to get the RPi to compile the thing (probably why it took such a long time to make).
http://wiki.samba.org/index.php/Samba4/HOWTO
Yours sincerely,
Jan Blomqvist Kinander