It might "officially" be 4K, but it's so heavily compressed that there are visible 8x8 blocks in the images, so it might as well be 8 times smaller - i.e. it has an effective resolution of 512x384
Take a look: http://imgur.com/YFsGb.png (I didn't change the image at all, except to crop it. I used mplayer to play it directly to png, and I made it a png so it's not jpeg artifacts.)
Interestingly the png of the 1080 is twice as large as the 4k version! Even though the 4k "officially" has twice the resolution in both directions (i.e. 4 times as many pixels).
Take a look: http://imgur.com/YFsGb.png (I didn't change the image at all, except to crop it. I used mplayer to play it directly to png, and I made it a png so it's not jpeg artifacts.)
For comparison here is the 1080 version: http://imgur.com/wkClm.png
Interestingly the png of the 1080 is twice as large as the 4k version! Even though the 4k "officially" has twice the resolution in both directions (i.e. 4 times as many pixels).