I think .mp4 is not supported natively. I had a similar problem yesterday. Try to convert your MP4 file into webm and ogg (to support all browsers).
You can then use
VideoElement e = new VideoElement();
print("canPlay: " + e.canPlayType('video/mp4'));
print("canPlay: " + e.canPlayType('video/webm'));
print("canPlay: " + e.canPlayType('video/ogg'));
which gives me:
canPlay:
canPlay: maybe
canPlay: maybe
to detect what formats are supported. MP4 is not supported.