Firefox 4 general release schedule is only after the Fedora 14 release and hence it won't be in the official repository. Tom Callaway has a repository available for Fedora 14 users 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