How does upgrading Adobe software work?

I'm thinking of upgrading from 4 to 5.5, and was wondering how the process works. Do I have to have 4 installed first, and then have 5.5 overwrite it? Or can I simply have a clean OS install, pop in the 5.5 upgrade DVD, and it will ask me for the 4's keys, after which 5.5 key, and then install 5.5?