Thank you ralvarezt, your fix Releases · ralvarezt/aics4_yosemite_fix · GitHub works perfectly! I am sincerely grateful.
For the rest of you. I had to temporarily go into System Preferences / Security & Privacy and check the "Allow apps downloaded from: Anywhere" button (which I unchecked again as soon as I had installed the fix).
Then I restarted Illustrator CS4 and lo and behold, it works as though there had never been a problem. Fantastic, I've got my Mac back!
