Hello:
Rather than adding a second server , join it to the domain, transfer FSMO roles to this domain, and drop the first controller, I think I would set up a clean 4.0 server and finally copy /var/lib/private from the old to the new machine. I cannot guarantee it is going to work, but w2orthy giving it a try. Rermber that all boxes must be configured exactly the same, same hostname, same domain, same modules, etc.
In addition to this, in order to move email, as emails are actually stored on /var/vmail (the storage for IMAP) I would suggest using imapcopy or rsync to copy them. Probably your users will need to set up a new profile in Outlook, but looks worthy to try.
Remember that if you wanted to move calendars/events/addressbooks you will need to make a backup of sogo database and import it into the new server