What is MP4?

The Mpeg4 format, if I understand it correctly, is a compression format used for video.

The way it works, is that it only reads the changes in a video frame and uses that portion in conjunction with the previous frame to generate the next frame in a video sequence.

The result is that by using only the changes, the file size will be much smaller. Say for instance, you have a background with a talking head' delivering the news on a set. Since most of the background doesn't change, there's no reason to re-write the entire frame in what hasn't changed, but just add and interpolate what has changed