MP4 is often referred to as MPEG-4, MPEG-4 Part 1, MPEG-4 Part 14 or MPEG-4 Part 12. Common extensions for MP4 files are .mp4, .m4a, .m4p, .m4b, .m4r and .m4v.

MP3 is short for "MPEG Audio Layer 3". MPEG, in turn, is short for "Motion Picture Experts Group," the organization that invented the compression algorithm and defined the spec. The number 3 means its the third-generation of the spec. The origin of MP4 is similar, but it's a newer spec, and includes video.

mpeg 4

It depends on the movie and receiver. The formats are either MPEG-2 or MPEG-4. Our HD receivers are in MPEG-4 and the other receivers are in MPEG-2. Eventually they will all be in MPEG-4.

Not necessarily. Although the most common MPEG audio format, there are also other standards, both previous to and successive to MP3. MPEG Audio Layer 1 (MP1) and MPEG Audio Layer 2 (MP2), as well MPEG-4 Part 3 could be classified as an "MPEG audio file."