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

A.
The native NT4 SAM replication protocols have not yet been fully implemented

B.
The database backends used by each server are incompatible

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

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