I’ve upgraded two systems at work from Fedora Core 3 to Fedora Core 4: a desktop using the normal installer, and a test server upgrading with yum. The yum upgrade worked well except for two snags. The first was a conflict with the old kernel-utils package. I followed the recommendation by installing the new kernel first, rebooting, then removing the old kernel.
The second was that SELinux denied access to about a dozen services on start-up. It was in auditing mode, not enforcing mode, so the services still worked, but I wanted to be able to start enforcing the policy once I resolved some other issues.