OS/X needs an uninstaller.
If you have an intact copy of your iPhoto 4 iPhoto Library folder, you can go back to the that version. Just delete the current applications and all files with iPhoto in the file name that reside in the HD/Library/Receipts folder. Also move your current iPhoto Library folder from the Pictures folder to the desktop. Then install iPhoto 4 from your CD, make any upgrades to it immediately before launching. Then copy your version 4 iPhoto Library folder into your Pictures folder. At this time you should run Disk Utility and repair disk permissions. Also delete the iPhoto preference file, com.apple.iPhoto.plist. Then launch the new iPhoto.