I found this very enlightening. It seems that Halo 4 was built with the Halo 3 engine with upgrades instead of Reach. At least, that's the impression that I got from the videos. It's possible that they simply threw the Halo 3 first-person skin on the player in a Reach engine. However, I've often felt like Halo 4 was more akin to H3, especially in Forge and Theater where the limitations no longer existing in Reach were present once more.
Not saying that it's a bad thing. I just think it's interesting and I probably would've done the same thing if I was making a sequel. :)
Great post, Mortem!