from Microsoft Support:-
"MPEG-4 is an International Standards Organization (ISO) specification that covers many aspects of multimedia presentation including compression, authoring and delivery. Although video compression and file container definition are two separate and independent entities of the MPEG-4 specification, many people incorrectly believe that the two are interchangeable. You can implement only portions of the MPEG-4 specification and remain compliant with the standard.
The MPEG-4 file format, as defined by the MPEG-4 specification, contains MPEG-4 encoded video and Advanced Audio Coding (AAC)-encoded audio content. It typically uses the .mp4 extension. Windows Media Player does not support the playback of the .mp4 file format. You can play back .mp4 media files in Windows Media Player when you install DirectShow-compatible MPEG-4 decoder packs. DirectShow-compatible MPEG-4 decoder packs include the Ligos LSX-MPEG Player and the EnvivioTV."