"Proper" 4K resolution is (QFHD, 16:9) 4096 x 2160.
This monitor uses UWQHD (21:9) resolution 3440 x 1440.
So not the "full" 4K resolution. However, this is good for the frame rate in games, as fewer pixels have to be calculated. 3440 x 1440 is still a very nice resolution.
HDR is High Dynamic Range, which has nothing to do with the resolution :)