If you are not planning on using your RHL9 system anymore, you can update to FC4 (or 5 which was released not too long ago). You won't even have to worry about that.
If you do plan on using the RHL9 system and the people with accounts you want to move over will continue to use the RHL9 system, you will need to do more and less than moving/copying some files. You could use one of the various remote verification systems to verify users who are logging in, without keeping that information on any more than one specific system. Using something like this, password changes will apply immediately to all systems using this method.
If you are still determined to just copy the account information, you will want to copy over your users' directories, which contain user settings. Traditionally, you would only need to copy over /etc/passwd, but most systems now have shadow passwords enabled, meaning that password hashes are no longer stored in /etc/passwd, but in a different file that can only be accessed by root. On many systems that use shadow passwords, the password hashes are stored in /etc/shadow.
NOTE: Account information for individual user accounts for different daemons (i.e., non-local accounts) may be stored in different locations. For the specific locations of that info, check `whereis daemon-name` or `man daemon-name`. Just copying over that information probably will not work. Fedora Core 4 has different software and more recent versions of much of the same software, much of which uses different file formats.
If you do plan on using the RHL9 system and the people with accounts you want to move over will continue to use the RHL9 system, you will need to do more and less than moving/copying some files. You could use one of the various remote verification systems to verify users who are logging in, without keeping that information on any more than one specific system. Using something like this, password changes will apply immediately to all systems using this method.
If you are still determined to just copy the account information, you will want to copy over your users' directories, which contain user settings. Traditionally, you would only need to copy over /etc/passwd, but most systems now have shadow passwords enabled, meaning that password hashes are no longer stored in /etc/passwd, but in a different file that can only be accessed by root. On many systems that use shadow passwords, the password hashes are stored in /etc/shadow.
NOTE: Account information for individual user accounts for different daemons (i.e., non-local accounts) may be stored in different locations. For the specific locations of that info, check `whereis daemon-name` or `man daemon-name`. Just copying over that information probably will not work. Fedora Core 4 has different software and more recent versions of much of the same software, much of which uses different file formats.