Firefox 4 is scheduled to be released only after the Fedora 14 release and hence Firefox 4 won't be in the official repository for that release. Tom Callaway has a repository available for Fedora 14 and Fedora 13 users that has the latest release however. Download http://repos.fedorapeople.org/repos/spot/firefox4/fedora-firefox4.repo and place it in /etc/yum.repos.d.
Use PackageKit or run the following command,
yum install firefox4