Can Samba Be a Backup Domain Controller to an NT4 PDC?
No. The native NT4 SAM replication protocols have not yet been fully implemented.
Can I get the benefits of a BDC with Samba? Yes, but only to a Samba PDC.The
main reason for implementing a BDC is availability. If the PDC is a Samba
machine, a second Samba machine can be set up to service logon requests whenever
the PDC is down.