You could upgrade to FC5's fedora-release rpm and try to yum upgrade to FC5. This is how I upgrade my workstation. This should theoretically work, but your server might just totally blow up in your face when you do this. Make backups and pray.
Or you could use pleskbackup to make a full backup of your server and then wipe the server and install the new OS. More downtime, but more likely to work without problems.
Or if you have another server: use the Plesk Migration Manager to migrate to a new OS and Plesk install on the second server.