MIXED MODE DOMAIN

I work for a bank that still uses windows NT 4 servers due to limitations as per the banking application. The domain was recently upgraded from a native NT4 to a Win2003 domain.User authentication has become a major problem since users in the branch offices can no longer authenticate from their NT 4 serveres (Previously BDC’s in the old domain structure)but now have to do that from HQ. A lot of traffic is being passed across the WAN, is there a way to make the NT4 servers still act as domain controllers??