MP4 file links download instead of playing

I suggest you to make updates on your web-server instead of WordPress. You can set mime type for mp4 files to application/octet-stream.

Using .htaccess (Apache):

AddType application/octet-stream .mp4

For nginx check documentation

If you need to change mp4 mime type on upload, you can use this hook:

add_filter('upload_mimes', 'wpse_320375_remove_mime_types');

function wpse_320375_remove_mime_types($mimes){
   $mimes['mp4'] = 'application/octet-stream';
}