If you install Internet Explorer SP1 after installing Windows 2000 SP4, the option to uninstall Windows 2000 SP4 is removed from Control Panel / Add\Remove Programs.
This was done to prevent conflicts in files and libraries that would arise if you uninstalled SP4.
To uninstall Windows 2000 SP4, first uninstall Internet Explorer SP1. This will restore the option to remove Windows 2000 SP4.
If you wish, you can re-install Internet Explorer SP1 after you remove Windows 2000 SP4.
NOTE: This issue does NOT occur if Internet Explorer SP1 is installed before installing Windows 2000 SP4.