Can't guarentee that it will work.
You first need to install fedora-release 6.x and releasenotes 6x.
Download both to the same location and install both at the same time rpm -Uvh *.rpm
You should probably backup your original fedora-release and release notes (don't know what the notes are called on fc4)
Disable any 3rd party repos. You may also need to disable extras for the upgrade.
You may be better off with Jman's recommendation (no guarentee there either though)