4k is two times 2k;-)
if you have a movie filmed on celluloid it runs with 24 frames per second. each frame is photographed and digitized and mostly this was done in 2k resolution. full hd is 1920x1080 pixels per frame, hd is i think 1280x720 and a dvd is 720x576. 4k is 4096x2160 pixels per frame.
when dvd came out it was fantastic, but no comparison to a theater screening. now with full hd i'm totally satisfied with the quality, the digitized picture can show grain of the celluloid and my question was also about, what is the resolution of a celluloid frame? it's defiantely more than full hd, but to me it feels very acceptable and it's for home use anyway. the big screen is always better and demands more resolution. the full-hd format also introduced the 24fps playing of a movie and the losless audio tracks, so theres nothing to complain or to wish for more by me here.
but with the resolution comes also the depth of colour and it should make a difference if the celluloid is captured in 4k and then downscaled to full hd.
but the main part of my question was: if you have an original print, celluloid, then its analog and you can screen it loke it was intended. if you have a 4k restoration, then the analog movie becomes digital and you need also a digital projector.
i see this topic different than when it's about music analog vs. digital and think a 4k movie could be a blast.
so i can now come back to topic and just found today an offer,was only 25€, of "suspiria" in a mediabook, a blu ray which has the restored 4k version on it and after this discussion here i had to buy it and i'm so looking forward to it.
i hope it will arrive next week and this will be my first 4k overworked movie.
of course there are now real 4k blu rays out but for that you'll need obviously a screen which can display that.