I have finished most of the upgrade to Fedora Core 4 from Red Hat 9. This was relatively successful, as you are reading this now.
I did encounter some problems with this upgrade, and I am going to attempt to document them. My notes may not be that clear, as I only got 4 hours of sleep in the past two days, because of this upgrade.
Fedora Core 4 includes PHP5. This mainly worked for the scripts that I run, but I did have to do some modification in order to get pLog to work. It appears that when pLog is run within PHP5, it uses more than the 8Mb of memory allocated to scripts. While in PHP4, this was not a problem. I increased it to 16Mb and the problem went away most of the time. I am still seeing it periodically.
Also PHP5 is more strict on php syntax, so there are a lot of notices in the logs. I am working through these warnings now, and I hope to check something in soon.
Also, It looks like the version of php that is included with Fedora Core 4, does not include GD. To get gd, you should run "yum install php-gd".
This took me the longest to get working. I had been using cryus previously for my imap server, so I was assuming that the upgrade would be seamless. It looks like the db4 database format changed sometime between RH9 and FC4. I used the steps on this page to get the imap folders working correctly. This was a pain for my account, as I have a lot of nested folders. In fact, I haven't finished this step yet