ok so here's the dumbed down way of explaining this, imagine a piece of standard film, consumers shot 8mm or 16mm film and cinema shot 35mm and 65mm. if the movie was shot film, it's already in 4k assuming they used quality film and a really good digital reader. the biggest difference is in the overall sharpness as there is more physical room to expose on 65mm film and yes, in theory, it does have a higher resolution but really if we remember film is an exposure with no resolution then we could argue that the biggest improvement in cinema would actually be better digital readers and better playback systems then throwing in 4k.