There is one single bad thing: Fedora Core 4 is not supported. The Fedore Legacy project that supported older Fedora versions stopped some months ago because of lack of funding and their archives are off-line now.
So you either have to manually check for updates for your system and install them from source when necessary, or upgrade the total server to another OS with a longer support timeline.
or upgrade the total server to another OS with a longer support timeline
Got my first chance to talk to server technical support this afternoon (the company is 1&1, btw) and put the point of no-update support to them. They offered to re-init the server to FC6. The update began within 15 minutes!
Not as good as FC7, of course, but at least there is a chance of security updates for a lot longer than FC4.
Many thanks for your assistance, lammert.
PS RedHat FC6 - Comments, opinions welcomed About to take over a root server with FC6 - main points to note?