MPEG-4 Part 14, the standard ISO-BMFF container for video/audio on the web.
| 0x00000004 | 66747970 | ftyp | ISO-BMFF 'ftyp' box |
| 0x00000008 | 69736F6D | isom | a major brand, e.g. 'isom'/'mp42' |
MP4 is the dominant delivery container for streaming and download video, typically carrying H.264/H.265 or AV1 video with AAC audio. It is built from boxes; the first is 'ftyp' at offset 4, whose brand (isom, mp42, …) marks it as MP4.
Have a file that might be MP4? Identify it with the File Type Identifier — it reads the magic number locally, nothing is uploaded. Or browse all formats.