Yeah, 60fps would be awesome even if they would have to lower resolution a little from native 4K. Maybe to around 1800p, should be a piece of cake for One X.
If the X is capable of running Halo 5 at 4K 60 FPS then it would have no problem whatsoever running Reach at 4K 60 FPS.
Doesn't work like that. Reach is being emulated, Halo 5 is running natively on the Xbox hardware. Emulating a game is far more difficult and taxing on the hardware than running it natively.
Also Halo 5 is not true 4K. It uses a dynamic resolution that does hit native 4K but it doesn't stay there all the time, just like how the regular Xbox One runs Halo 5 at far less than 1080p in almost all scenarios. Xbox One X hits resolutions such as 1584p and 1890p, with it only doing native 4K (2160p) in uncrowded areas.