Ok, do exactly this:
1 - Download the drivers to your Desktop.
2 - Get out of x into a console mode.
Easy way to do that is load a terminal and type either init 3 or /sbin/init 3
3 - cd /whateveryournameis/Desktop (just cd to the Desktop, whatever directory it is)
4 - sh -i <name of the nvidia driver file on the desktop>
5 - pico /etc/X11/XF86Config (open the file in a primitive editor)
OR pico /etc/X11/xorg.conf I can't remember which one FC4 uses.
6 - Find "Section Device" and change the Driver to "nvidia"
7 - CTRL+X and save the settings.
The latest version of the nvidia drivers will walk you thru re-writing your xorg.conf file, so you shouldn't really need to do that
8 - lastly go back init x by using either init 5 or /sbin/init 5