using nt4 server as Backup domain controller to windows 2000 server

[kesm]

This is impossible isn't it?
There is no way to replicate the active directory on windows nt4.
We are looking to set-up a new win 2k server, and our current windows nt4 server will be demoted to a bdc. We can replicate the data directories between the two, as user authentication runs from another nt4 trusted domain with its own bdc. Can the two servers be set-up to replicate the data or does the current nt4 server require an upgrade to windows 2k server?

Thanks