SIP Domain support, sip.conf and user authentication

I want to have mutple users with same extension but on different domains.
i.e. I want:
1234@pbx1
1234@pbx2
1234@pbx3
1234@pbx4
4444@pbx1
4444@pbx2
4444@pbx3
4444@pbx4
so that each user can register with their own userid@domain with a password. Once registered, the @pbx1 users can call each other
by extension and the @pbx2 users can call themselves, etc. dial plan
stuff would handle users@pbx1 interacting with users@pbx2, etc.

I know that Asterisk 1.2.1 ( or somewhere around there ) added domain
support… and I think I can use SIPDOMAIN in the dial plans to aid in
doing the xxxx@domain stuff… but I can’t see how SIPDOMAIN or domains
in general affect the sip.conf file. I really, really, really want to avoid
having the users login as 1234_pbx1, 1234_pbx2, 1234_pbx3, etc. They
are really 1234@pbx1, 1234@pbx2, etc.

Thanks - jack