Samba supports simultaneous installs of samba3 and samba4. For awhile there will be simultaneous installs on "AD Controllers" since samba4 is in beta however it is the only way to do an AD controller so the users will probably use samba3 for everything else. Upstream supports simultaneous installs on the same prefix so this is safe http://wiki.samba.org/index.php/Franky.
*** This bug has been marked as a duplicate of bug 424633 ***