Mp4 is a common video container used to deliver videos. Video containers are used to send video & audio compressed with certain codecs. The mp4 video container is based on the QuickTime video container created by Apple, but has wider support across devices than the .MOV file (it also offers higher compression than the .MOV file does).
video codecs used with mp4
The most popularly used video codec is the h.264 codec, and that video is sent in the mp4 container. The h.265 video codec is also often used in the mp4 container.
audio codecs used with mp4
The most common audio codec used with mp4 is AAC (Advanced Audio Coding).
mp4 at api.video
Each video uploaded to api.video will have a h264 encoded mp4 file created (unless the mp4Support parameter is set to false.