After debraying, the red epic camera (5K that they used in the movie) has a correct ratio sharpness/resolution at 3.5k.
That being said, like almost every title on the big screen, a lot is done in 2K.
The extra resolution is usually here to help the post-production but has no benefit on the receiving end of the audience since they would need to sit much closer to the screen and so not see the entire screen (who does that?)
The picture is 4K, what's terrible in it is the bitrate, a lot informations has been comrpessed (whether it's a 2K upscale or a terrible jpeg comrpession)
It's by far even worse than the 1080p bluray which has much finer details.
If you want a good analyses on 2K, 4K, 6K, 12K IMAX, 35mm & co I suggest you take a look at Steve Yedlin on the resolutions:http://nofilmschool.com/2017/08/yedlin-camera-resolution-myths
(it's quite long about an hour, but very instructive).