An old camera's spec sheet says it records "MPEG-4". A forum answer says your video "is MPEG-4, which iPhones can't play". Your computer's file properties describe a perfectly ordinary .mp4 as "MPEG-4". And yet most .mp4 files play on an iPhone without any trouble. All of these can be true at once, because "MPEG-4" gets used for three different things - and only one of them is the reason a video won't play.
#The one-sentence version
MP4 is a file type: the .mp4 wrapper that holds video, audio, and subtitles. MPEG-4 is the name of a whole family of standards, and people use it to mean either that file type, an older video format called MPEG-4 Part 2 (the one inside DivX and Xvid files), or the modern H.264 format, whose official name is MPEG-4 Part 10. When an "MPEG-4" video refuses to play, the culprit is nearly always Part 2.
#Where the name comes from
MPEG stands for Moving Picture Experts Group, a committee set up in 1988 under the ISO and IEC international standards bodies to agree on how digital video and audio should be compressed. It numbered its standards by generation:
- MPEG-1 (1993) gave us Video CDs - and MP3, whose full name is MPEG-1 Audio Layer III.
- MPEG-2 (mid-1990s) became the format of DVDs and digital TV broadcasts.
- MPEG-3 was planned for HDTV, then folded into MPEG-2 before it was finished. It never shipped, which is why MP3 is not "MPEG-3".
- MPEG-4 (first published in 1999) is less a single format than a toolbox: more than 30 separately numbered "parts", each its own standard, covering video, audio, file formats, subtitles, and more.
So "MPEG-4" on its own is about as specific as saying a car is "a Toyota". Three of those parts are the ones you'll actually run into.
The three parts of MPEG-4 people actually mean
- Inside DivX, Xvid, and many 2000s camera and phone clips
- Shown as MPEG-4 Visual, mp4v, XVID, DIVX
- Plays in VLC and desktop players
- Chrome, Safari, and most phones won't play it
- The video format behind most video today
- Shown as AVC, avc1, H.264
- Hardware decoding in every phone, laptop, and TV
- Plays everywhere
- The wrapper, not the picture
- Relatives: .m4a, .m4v, .3gp
- Built on Apple's QuickTime format
- Can hold Part 2, H.264, HEVC, or AV1
MPEG-4 Part 2: the one that causes trouble
Part 2 (officially "MPEG-4 Visual") was MPEG's first video format for the internet era. It arrived in 1999, just as people started sharing video online, and became the engine inside DivX and its open-source twin Xvid - the formats behind most downloaded and DVD-ripped video of the 2000s. Many digital cameras and phones from that decade also recorded "MPEG-4" video, which almost always meant Part 2.
Part 2 lost out to H.264, which squeezes video about twice as well, and the industry moved on. Today Chrome, Safari, and Android's built-in video player have no decoder for it, and iPhones and iPads don't play the DivX and Xvid files that make up most of what's still around. VLC and most desktop media players still handle it, which is why these files tend to work on an old PC and fail almost everywhere else.
MPEG-4 Part 10: H.264 under its official name
H.264 is the video format nearly everything uses today: phone cameras (when they aren't using HEVC), YouTube uploads, video calls, streaming services, screen recorders. Its official MPEG name is MPEG-4 Part 10, Advanced Video Coding - AVC for short. "H.264" is the name the ITU, the telecoms standards body that co-wrote it, gave the very same standard. So when a tool lists a file's video as "AVC" or "MPEG-4 AVC", that's H.264, and it plays on practically every device made in the last 15 years. H.264 vs H.265 untangles the naming further.
MPEG-4 Part 14: the .mp4 file itself
Part 14 defines the .mp4 file format - the wrapper. It doesn't compress anything. It organises the compressed video, the audio, any subtitles, and an index that tells players where everything sits in the file. It was built on Apple's QuickTime format, which is why MP4 and MOV files are near-identical twins underneath (see MP4 vs MOV). The same family includes .m4a for audio-only files, Apple's .m4v, and the .3gp files old phones recorded. The audio inside an MP4 is usually AAC, which MPEG-4 covers in yet another part (Part 3).
The key point: an .mp4 file can legally hold several different video formats - H.264, the newer HEVC and AV1, and the old Part 2. The extension tells you what the box is, not what's in it. Codecs vs containers explains that difference in more detail.
One more twist: HEVC (H.265), the format newer iPhones record in, isn't part of MPEG-4 at all. It belongs to a later family called MPEG-H, even though it usually travels inside an .mp4 or .mov file.
#Why an ".mp4" can still refuse to play
Because the wrapper and its contents are separate, you can end up with a perfectly valid .mp4 that holds Part 2 video. Older converters produced these routinely, some cameras recorded them, and an AVI that's rewrapped as .mp4 without re-encoding carries its DivX video straight across. The file looks normal and the extension is right, but Chrome or an iPhone shows a black screen, plays sound with no picture, or won't open it at all.
Wave can produce one of these files too, and it's worth being upfront about when. In a browser with WebCodecs (Chrome, Edge, and Safari 16.4 or later), Wave writes H.264. In a browser without it, Wave falls back to an encoder that writes Part 2, because the popular open-source H.264 encoder (x264) is GPL-licensed and can't be bundled into Wave. When that happens Wave says so before you convert: the result plays in VLC, QuickTime, and most desktop players, but not in Chrome, Safari, or on phones. What is WebCodecs? covers why the browser matters.
#How to tell which "MPEG-4" you have
You need a tool that shows what's inside the file, not just its name. Three easy options:
- VLC (free, Windows, Mac, and Linux): open the video, then Tools → Codec Information on Windows, or Window → Media Information on a Mac.
- MediaInfo (free): open the file. The "General" section describes the wrapper; the "Video" section describes the compression.
- Wave: drop the file onto any converter page. The inspect screen shows the video and audio formats before anything gets converted.
What the label says, and whether it plays on phones and in browsers
| The tool shows | What it means | Phones & browsers |
|---|---|---|
| MPEG-4 (General / container) | The .mp4 wrapper (Part 14) | Depends on the video inside |
| MPEG-4 Visual · mp4v · XVID · DIVX · DX50 · FMP4 | Part 2 video | No |
| AVC · avc1 · H264 - MPEG-4 AVC (part 10) | H.264 video | Yes |
| HEVC · hvc1 · hev1 | H.265 video (MPEG-H, not MPEG-4) | Mostly Windows 10 needs an add-on |
| AAC · mp4a · MPEG AAC Audio | AAC audio (Part 3) | Yes |
#Fixing a Part 2 video
Changing the wrapper alone won't help: an AVI rewrapped as MP4 still has Part 2 inside. The video has to be decoded and re-compressed as H.264. That takes longer than a wrapper swap - minutes rather than seconds - and costs a very small amount of quality, but because H.264 is about twice as efficient, the file usually gets smaller too.
Wave won't simply rewrap Part 2 video into an MP4, because the result wouldn't play in the browser you made it in. In a browser with WebCodecs it re-encodes the video to H.264 instead, on your device, without uploading anything. Use DivX to MP4 or AVI to MP4 for files from the DivX era, 3GP to MP4 for old phone videos, or the main converter for an .mp4 that already has Part 2 inside.
#Quick glossary
- MPEG-4: a family of more than 30 standards. On its own, ambiguous.
- MP4 (MPEG-4 Part 14): the .mp4 file format - the wrapper.
- MPEG-4 Part 2 (MPEG-4 Visual): the older video format inside DivX and Xvid files. Poor support on phones and in browsers.
- H.264 (AVC, MPEG-4 Part 10): the modern video format. Plays everywhere.
- HEVC (H.265): the successor to H.264. Not part of MPEG-4, but usually inside an .mp4 or .mov.
- AAC: the usual audio format inside an MP4.
- M4A, M4V, 3GP: members of the MP4 family for audio-only files, Apple video, and old phone video.
- MP3: MPEG-1 Audio Layer III. Not MPEG-3, and not related to MP4 beyond coming from the same committee.
#The takeaway
MP4 is the box. MPEG-4 is the family name, and it gets attached to three different things: the box itself (Part 14), the modern H.264 video that usually goes in it (Part 10), and the older Part 2 video from the DivX era. If a video labelled "MPEG-4" won't play on a phone or in a browser, check the video format inside. If it says MPEG-4 Visual, DivX, or Xvid, it needs re-encoding to H.264 - a new file extension won't fix it.