There are lots of ways are available for converting a video into mp4. But you have asked for the easiest so I will tell what is the easiest and the fast way to convert youtube video to mp4. There is a website called fbtube. Which is launched for downloading & converting videos in mp3 mp4 and several other formats from Youtube as well as Facebook You can try that because I can bet that there no other way to convert a youtube video in mp4 as fast as it does.