The developers explain that it is possible to update from the 3.x branch
Samba, an app that seamlessly integrates Linux/Unix servers and desktops into Active Directory environments using the winbind daemon, is now at version 4.0 RC4.The release of the forth release candidate in the 4.x branch contains all of Samba's technology parts, both a file server, which can be upgraded from existing Samba 3.x releases, and the AD domain controller previously known as ‘samba4.’
Users have to know that it is possible to upgrade from Samba 3.x to Samba 4.x and they must use 'samba-tool domain classicupgrade' command.
Users upgrading from previous versions in the 4.x branch, should run 'samba-tool dbcheck --cross-ncs --fix' before re-starting Samba. Users upgrading from earlier alpha releases should contact the team for advice.
Highlights of Samba 4.0 RC4:
• Samba 4.0 now supports the server-side of the Active Directory logon environment used by Windows 2000 and later;
• The Domain Controller (DC) implementation includes a built-in LDAP server and Kerberos Key Distribution Center (KDC), as well as the Samba3-like logon services provided over CIFS;
• Samba 4.0.0rc4 ships with two distinct file servers. It is now possible to use the file server from the Samba 3.x series 'smbd' for all file serving by default;
• The latest version also ships with the 'NTVFS' file server. This file server was used in all previous releases of Samba 4.0.
• The binaries users would expect from that series (nmbd, winbindd, smbpasswd) continue to be available;
• The developers provide two DNS solutions, a simple internal DNS server for 'out of the box' configurations and a more elaborate BIND plugin using the BIND DLZ mechanism in versions 9.8 and 9.9;
• A new scripting interface has been added, allowing Python programs to interface to Samba's internals.
Check out the official announcement for a detailed description of the update process and various other changes in the software.
Download Samba 4.0 RC4 right now from Softpedia. Remember that this is a development version and it should NOT be installed on production machines. It is intended for testing purposes only.