Upload MP4 to WordPress Media library… example Added here as Video Block item (modern WordPress).
Or use custom HTML:
<video id="m1" width="100%" height="100%" autoplay loop muted> <source src="http://cogdogblog.com/wp-content/uploads/2011/06/google-guitar-hero.mp4" type="video/mp4"> Your browser does not support the video tag. </video>