What's the relationship between yum and rhnplugin?

I know RHEL 5 uses yum and rhnplugin to access RHEL 5 RHN.

My question are:
(1) Can I use yum+rhnplugin to access RHEL 3/4 RHN? Is there any
difference between "RHEL 3/4 RHN repository" and "RHEL 5 repository"?
(2) Can I use yum+rhnplugin to access RHN's counterpart 'current'
(current.tigris.org)?
(3) I assume 'RHEL 5 RHN' is still a 'RHN repository', not a normal
'yum repository' created by 'yum-arch' or 'createrepo'. I mean Red Hat
only changes the update *client* from up2date to yum, but does not
change the repository from RHN repository to yum repository.Am I
right?

Best Regards,
Xie, Bo