I thought people would like to know that AuraSoldier313 has completed Halo 4 on Mythic. This claim might sound a little dubious, but based solely on the videos he has posted so far, I think it's valid. You can see his completion post in the official Mythic thread on Waypoint here:
And here's his Halo 4 Mythic playlist from his YouTube channel, where he's posted several of his Mythic runs already. According to him, the rest are incoming:
Seriously, check some of these runs out. His Knight takedowns are incredible.