From Fedora Project Wiki

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 and place it in /etc/yum.repos.d.

su -c 'wget -P /etc/yum.repos.d/'

Use PackageKit or run the following command,

su -c 'yum install firefox4'

Note: Some (if not all) versions of these Firefox 4 betas include versions of xulrunner2 which may be incompatible with other software in your system. While either firefox(3) or firefox4 will run as expected (serially, not concurrently), attempts to use System, Help may vanish with no visible results. See [1].