.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.