It's 4k using the same resolution as 4k movies,
Those 4k marvel films, and others are 3840x1640, when viewed on a 16:9 screen you see the black bars for the wide-screen crop, they are included in the 2160p measurement but the actual film, is 1640p.
In cinemas too, it's 1640.
The 4k measurement is taken from the 3840 pixel count on the horizontal, so by accepted cinema standards, 3840x1640 or above, is 4k cinema standard display.
2160p at 21:9 would actually be a 5k display (5040x2160)
