MP4 files often present compatibility issues due to their audio format, primarily AAC, which is not recognized by some players like DivX. Users report difficulties playing MP4s with Windows Media Player 9, Real Player, and QuickTime, with many recommending the use of updated codecs or alternative players such as PowerDVD and Nimo Codec Pack. Conflicting codecs can lead to playback problems, so it is advised to minimize codec installations and use tools like GSpot to identify necessary codecs. Some users have successfully played MP4 files by ensuring they have the correct codecs installed or by using specific software that supports MP4 playback. Additionally, issues with video playback, such as a blue screen, may indicate codec conflicts or file corruption. For those needing to convert MP4 files to AVI, various conversion tools are available, and users are encouraged to seek reliable tech support for further assistance.