@Bas: According to Wikipedia digital movies are usually mastered in 2K (2048x1024), so that would seem to be a limiting factor in the usefulness of 4K or higher displays.
That same article also says that the resolution of film is hard to determine, but probably effectively quite a lot higher than 2K, although projected analogue film in theatres probably have far less resolution due to duplication and limitations of the projection system.
Though I have to say, I saw the Hobbit last Saturday, and there were a few places (particularly the (Japanese) subtitles) where the limited resolution of the digital projection was really noticeable. Mind you, we didn't make reservations so we were sitting in row 3, quite close to the screen.
Then again, I always thought regular analogue projection looked blurry, and at least digital film doesn't have the problem that if you see a movie two months after it opened the print is full of scratches and dirt.