Do not upgrade your Fedora Core 4 kernel to 2.6.15
I recently ran "yum update" on a bunch of machines at work. They were all "new" machines, and were not doing much anything interesting yet. Which is a good thing.
Apparently, somewhere between 2.6.11 and 2.6.15, some stuff with "udev" and partition labeling changed, because none of these systems would boot any longer. There were two problems: the first, which was that using "root=LABEL=/" in the grub boot loader no longer works. That was easy enough to fix by booting the old kernel and editing the grub.conf. The second problem is that the boot sequence needs /dev/console, which is no longer available early on in boot sequence on kernels that are "pure" udev. Whatever that means.
So my free advice is don't run "yum update." Instead, run "yum --exclude=kernel update"