Lets all take a second to think about this logically (except you stinkles, you already know whats happening)
If it was just a 4k update to Halo 5 Phil just would have announced it on stage...it would be no different then gears 4 other than slightly less expected because gears shipped so recently
So it has to be related to MCC and some overhaul therin...because thats news actually worth delaying...
Or i'm wrong and they will just announce halo 5 4k at an event along with halo 6 trailer and it was just delayed because phil didn't want to say halo during a presser for the first time in 10 years (

)