Samba cannot be a Backup Domain Controller for an NT4 PDC because:

A.
Samba 3 implements 128-bit encryption, which is unsupported by an NT4 PDC.

B.
the native NT4 SAM replication protocols have not yet been fully implemented.

C.
the database backends used by each server are incompatible.

D.
the Samba team does not want to encourage the use of a Windows server as a PDC.