Container vs codec
MP4 (formally MPEG-4 Part 14) is a box, not a codec. It holds a compressed video stream, audio streams, subtitles and metadata, all interleaved. The actual compression is done by a codec inside — most often H.264 (AVC) for video and AAC for audio, with newer files using H.265 (HEVC) or AV1.
This is why "MP4" alone doesn't guarantee a file plays: a device has to support the codec inside, not just the MP4 wrapper. In practice H.264-in-MP4 is the most universally supported combination there is, which is what gives MP4 its reputation for "just working".
Why MP4 is the safe choice
When an upload form, editor or player rejects a video, converting it to MP4 (H.264) is the usual fix. It streams well, hardware-accelerates on most devices, and balances quality against size sensibly. More flexible containers like MKV can hold more, and WebM is favoured for open-web delivery, but for sheer "send it and it'll play", MP4 is hard to beat.
Strengths
- Plays on virtually every device, browser and platform.
- Hardware-accelerated H.264 decoding is near-universal.
- Good balance of quality, size and streaming support.
Limitations
- Playback still depends on the codec inside the container.
- Common codecs (H.264/H.265) carry patent licensing.
- Less flexible than MKV for multiple tracks and subtitles.
When to use MP4 video
Use MP4 (H.264) whenever you need a video to play reliably everywhere — uploads, sharing, embedding. Reach for WebM for open-web delivery or MKV for rich multi-track archives.
MP4 video FAQ
- Why won't my MP4 play?
- The container is fine, but the codec inside may not be supported by your player — for example H.265/HEVC on older software. Re-encoding to H.264 in an MP4 almost always fixes it.
- How do I convert MKV or AVI to MP4?
- Transcode the video to H.264 in an MP4 container. Morphr does this in your browser, re-encoding the picture and sound so the result plays on standard players.
- Can I get just the audio from an MP4?
- Yes — extract the audio track to MP3, M4A or WAV. Morphr does this locally, keeping the sound and discarding the video.