The iPhone 4+ does play of MP4 videos… Safari is a browser which runs on any device (iPhones, iPads, Macs, Windows, etc).
There is a known problem with older version of Safari on iphone where it doesnt playback certain resolutoins of video in the browser sessions (notes here: https://goo.gl/KZV6kf
) The work around is to (1) download the video to your iphone and then play it locally or (2) install a media player that support all video formats… here is one https://www.ehow.com/how_5748981_watch-mp4-iphone.html
The MP4 video format is compatible with the iPhone, meaning you should be able to play video files without needing any conversion or re-encoding. For many situations, the native Video application on the iPhone will be fine for watching an MP4 file, whether on your phone or your computer. However, several independently developed apps can give added functionality.
Read more : https://www.ehow.com/how_5748981_watch-mp4-iphone.html