samarn is correct but I'm wondering if the MP4 file had been renamed. I used to have to change the suffix of files to get them to play in some players or Camtasia. E.g. 3gp to mov; mov to mp4, etc.
One way to know if it is really an MP4 file is to look at the header. One created by CS8 contains information such as "ftypmp42"; "isommp42" at the start and a bit later it contains "Mainconcept MP4 Video Media Handler".