Joining a Samba DC to an Existing Active Directory
From SambaWiki
Samba4 joining a domain
As of Samba4 alpha11, Samba4 now has the ability to join an existing Active Directory domain as an additional domain controller. The process of joining a Samba4 server to an existing domain is a bit different to provisioning a new domain. This process is the equivalent of the 'dcpromo' command on Windows servers.