First, it is not a good idea to jump to the latest release of a linux distribution a few days after the release when you run a server! Why do you want to do that when your server runs fine now? Fedora 4 will be supported with fixes some time.
The second is, Fedora 5 is not supported by ISPConfig yet. If you run the ISPConfig installer on Fedora 5, the installer will stop with the message like "unsopported operating system".