Ok, Moodle 2.2.x can be hacked by adding mp4 to mediafilter,
But Moodle 2.3.x changed the entire media filter. I think you just hack the filelib.php and change the
'mp4' => array ('type'=>'video/mp4', 'icon'=>'mpeg', 'groups'=>array('video','web_video'), 'string'=>'video'),
to
'mp4' => array ('type'=>'video/x-flv', 'icon'=>'flash', 'groups'=>array('video','web_video'), 'string'=>'video'),
Hoping this doesn't mess too many things up. Very few people use a browser that does HTML5 correctly and flash video players work MUCH better for your average user.