First off, why convert to an MP4? The rationale behind this is that most modern browsers support MP4 videos, almost all other video types will require you to download a browser extension to view. So, to put it simply, it's the most universal video format. Depending on how you create your video, most programs will give you the option of exporting in the mp4 format. If not, you can do the following:
Use iMovie. Simply create a new Event and then import your video. Once the video is imported, select "share". The resulting file should be an mp4.
The Video Converter Android app allows you to convert almost any file to mp4.
You can get a program or now app for windows or visit Online Converter and have them do it for you.