Originally Posted by zazenmac
I have an mp4 music video in my itunes library and it plays fine, but when i drag it onto my iphone it says it cannot be played on this iphone, do i need to save it to a different file format?
MP4 only defines the wrapper file format. It does not deal with the actual video codec being used. IIRC iPhones can only decode the MPEG-4 Part 10 codec (otherwise known as H.264).
MP4 files may commonly contain video encoded with MPEG-4 Part 2 (eg. DivX), MPEG-2 (eg. DVD video), or MPEG-1 (eg. VCD). Less commonly, virtually any video codec may be embedded within an MP4 container, from Intel Indeo to RealVideo to Windows Media.
You may want to start by determining what video codec is actually being used within the MP4 file.