SvennoJ
4096x2160 and 2048x1080 are just projector standarts, the actual movie res. is different and depends on Aspect Ratio, that's why home 4K standart is 3840x2160(1.78:1)
http://en.wikipedia.org/wiki/Digital_Cinema_Initiatives
Just if you didn't know.
In theater, even 2048x858 movies looks great today.