A file with extension MP4 is a MPEG-4 multimedia file format. 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. You can play back .mp4 media files in Windows Media Player when you install DirectShow-compatible MPEG-4 decoder packs. Microsoft has chosen to implement the video compression portion of the MPEG-4 standard and has currently produced the following MPEG-4-based video codecs: Microsoft MPEG-4 v1, Microsoft MPEG-4 v2, Microsoft MPEG-4 v3, ISO MPEG-4 v1.