NT 4.0 and 2000

I have a small LAN with two servers running NT4.0. One server is acting as a “PDC” and the other is a member server with its own domain, which only runs our phone system. My question is: If I upgrade the “PDC” to Windows Server 2000 can I still run NT4.0 on the second server? I will eventually upgrade the second server to Windows 2000 but I have to take things one step at a time.