Please remember that 4k is literally 4x as many pixels as 1080p, and while it is very imprecise to say that gpu performance cost is x effect * y pixels, it's not that far off from true for most modern game engines.
A respectably powerful gpu that's a third as powerful as a very powerful gpu on the same platform is reasonably safely going to be able to run a game at 1/4th the resolution, and since gamers and tv manufacturers have marketed 4k as a must have feature i can't imagine much being "held back," especially when you consider the ubuiquity of expensive, scalable elements like shadows, motion blur, dof, volumetrics, and other effects that render to arbitrary resolutionn buffers which could be turned down further if necessary without hurting the art direction of a game..
It is a nail in the coffin for everybody out there (me!) hoping more developers would abandon 4k and use the extra headroom to make a *vastly* better looking 1080p game though.