The real key is what is inside the MP4 files
Read Bill Hunt on a file type as WRAPPER http://forums.adobe.com/thread/440037
What is a CODEC... a Primer http://forums.adobe.com/thread/546811
What CODEC is INSIDE that file? http://forums.adobe.com/thread/440037
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Report back with the codec details of your file, use the programs below... a screen shot works well to SHOW people what you are doing
http://forums.adobe.com/thread/592070?tstart=30
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For PC http://mediainfo.sourceforge.net/en or http://www.headbands.com/gspot/