Join an Existing Site

I have an Exchange Server 4.0 currently running. I want to add an Exchange 5.5 server to our current site, move the mailboxes over then remove the 4.0 server. When I goto add the 5.5 server to the site I get an error message saying the 4.0 server is licensed for stand-alone use only. Install Microsoft Exchange Server Connector on the 4.0 server if you want to join this site.

I don’t know where to get this connector or how to install it. I have an Exchange Server 4.0 disk and an Exchange 5.5 server disk.

Thanks