Samba 3 and Samba 4 can be run at the same time safely. It might be nice, especially because the beta for samba 4 is out, to make slots for samba 3 and samba 4 so that users can build both.
*** Bug 427988 has been marked as a duplicate of this bug. ***
This is not a straightforward thing to do. Samba 3.6 and samba 4 share some libraries ie tevent, ldb, tdb.
So I have to check if it is possible that many different versions of those libraries can live in the same system.
I checked with upstream, it's possible. In other news I'm about to submit a samba 4 ebuild.
I think this shouldn't be done. Samba 4 is the next version and includes *both* samba 3 code and the AD code and all users should migrate when it's stable. See the discussion in Bug #195703. Especially comment 137 and 138. It should be solved in init scripts and configuration files, not splitting into two packages.
This is not going to happen
Samba 3 is deprecated
Samba 4 will be stabilized soon.