you can probably change your repositories for your package manager to search for FC5 software, rather than FC4, as it is set up now. read this page
, specifically 2.2 and 7.*.
use YUM to view your current repositories... somewhere in the link(s) that it shows, it will have a '4' representing FC4.. most likely all youll have to do is change that to a '5' wherever you see '4'. save the respositories, update package list, and upgrade system. (note this is my assumption, i dont use yum or FC so im not guaranteeing anything)
heres what a quick search brought up: http://www.brandonhutchinson.com/Upg..._with_yum.html
if that doesnt work or you dont want to try that, search somewhere else for upgrading fc4 to fc5.. good luck!