Quicktime 7.4 not playing MP4s

Ever since I downloaded 7.4 MP4 files stopped playing. I've looked through the other topics and nothing helped me. It worked fine for 7.3. Not all MP4s don't work though. Only ones I downloaded from YouTube using TubeSock. ISQUINT'S DEBREAKER DOESN'T WORK AT ALL. It just comes up with "Applescript error: NSReceiverEvaluationScriptError: 4 (1)". I want to downgrade to 7.3, and I downloaded Pacifist, BUT THAT ALSO CAME UP WITH AN ERROR. What should I do.