Halo 4 actually does significantly scale its graphics by gameplayishness of area. There's a difference of astronomical proportions between the start of Requiem, and the first real combat area of the level. And not just in terms of basic design stuff like environmental texture quality and polygon counts; there are dynamic snazzinesses like water splash effects that seem cut down as well.

And to its credit, the game does focus on maintaining a reasonably solid 30fps in campaign. The same can't be said for Reach... although, something like the exterior of New Alexandria is a billion times more technically ambitious as gameplay area design than anything in Halo 4.