Use the Install/uninstall utility that is built into the OS.
On the mc it is called Add or Remove Adobe Illustrator CS 4 you can also, if that does not work go to the Adobe Site and search for the Adobe CS4 Clean up script.
After you run the script you have to manually remove things like settings folder the Illustrator application folder, there should be instructions when you download the script.
If you had to activate the trila pease deactivated it first.
As Wade said, the programs use regular uninstallers. On Windows, this is in the Add or Remove Programs section of your system control panel, on Mac you should have an alias inside the application folder that reads Remove/ Uninstall Illustrator CS4. I'm not sure if you ever will be able to install as an upgrade, though. For CS4, the last qualifying version is CS2, not CS.
Many thanks for your help. All sorted.
The updates are for valid CS upward.
We are glad it all worked out.