It really is that simple to upgrade, there is no documentation other than that on the main website page. Restoring, again, is fairly simple and generally painless. If you've taken a backup then you should have no worries about the upgrade or getting back to a working system should it fail, that's what the forums are here for - to help. I can give you all the reassurances in the world but you'll have to do it one day.
Restoring a system has been covered many times in the forum, didn't you come across it? Restore the files, run (as root) zmfixperms, install the same version you had on the backup and you should (with a following wind) be up and running.
The version you have is extremely old and you should upgrade as soon as you can, there have been many improvements since that version was released.
I missed the bit about you having FC4 Zimbra installed on an FC5 system, you should also get out of that set-up asap. Do you have an additional server that you could use as a test machine to migrate your current set-up to? What's the specifications of your current server? On a personal note, I'd rather you moved off Fedora and onto something like CentOS as the operating system.