Not to nitpick but this is untrue. Very very few screens are showing in true 4K at this moment in time. There are hundreds or maybe even thousands of the Sony Dual lens 3D systems in theaters across america that say they are 4K in the Sony branded pre-roll in the theater. These are really projectors that have 2 x 2K projection systems in one projector body. They are usually projecting from 2K (2D) DCPs or 2K 3D DCPs that are actually split between each lens/sensor and projected interleaved (which does not actually give you anywhere near 4K projection). Additionally, the current Digital Imax system does the same thing with two completely separate interleaved 2K Christie Cinema projectors projecting from a 4K DCP. This translates to a very, very slight increase in detail over a single 2K system (and is done to give each eye more light). This translates to very few true 4K DCPs being projected on true 4K projectors. Even in specialty situations where we have a movie like Prometheus that was captured at 5K native. I believe I read that Prometheus was posted as a full 2K DI. The Visual effect sequences were definitely rendered in 2K. So this means that even when they had specialty 4K screenings at select locations those were partially or completely 2K content on a 4K system.
Even in a true 4K situation, such as when I saw Samsara projected at the Arclight Hollywood in 4K it was stunning but not nearly as much of an improvement over 2K (in a high end theater setting mind you) as you would hope. And this is coming from a guy with 20/10 vision who has overseen dozens of premieres and special screenings (many of them at the arclight). 4k is amazing but it's not as amazing as it should be or as prevalent as you are led to believe.
The single most stunning resolution experience I have seen was the Dark Night in Imax at Universal City. Those prints were supposedly struck from the negatives. This means the viewer was seeing something between 8k and 16k on the screen during the imax sequences. THAT was what I always hoped true 4K would look like. Someday.