.mp4 — MPEG-4 Video
Video video/mp4
What a .mp4 file is
The default container for video on the web and on phones, normally holding H.264 or H.265 video with AAC audio.
How to open it
Everything.
Worth knowing
MP4 is a container, not a codec. A file that will not play usually has an unsupported codec inside — often H.265 on older hardware.
How to identify a .mp4 file
A file's extension is only its name — renaming a.png to a.mp4 changes nothing inside. What software actually reads is the magic number, the first few bytes of the file.
| Signature | ftyp at offset 4 |
| MIME type | video/mp4 |
| Category | Video |
ISO base media container; the brand that follows says which flavour.
# what is this file really?
file mystery.mp4
# look at the first bytes yourself
xxd mystery.mp4 | head -2
head -c 16 mystery.mp4 | xxd
# Windows PowerShell
Get-Content mystery.mp4 -AsByteStream -TotalCount 16 | Format-HexFrequently asked questions
What is a .mp4 file?
The default container for video on the web and on phones, normally holding H.264 or H.265 video with AAC audio.
How do I open a .mp4 file?
Everything.
Is .mp4 safe to open?
Yes. A .mp4 file is data, not code — opening one cannot execute anything by itself.
Can I change a file's extension to convert it?
No. Renaming changes the label, not the bytes. The one place it appears to work is between formats that genuinely share a container — renaming .docx to .zip works because a DOCX is a ZIP. Everything else needs a real converter.