In 4K Mode the game will run in 4K resolution (or supersampled on a non-4K display) with stable 30fps gameplay, whereas Performance Mode enables dynamic resolution scaling (from 1080p up to 4K) while targeting 60 fps during gameplay. Both modes support HDR.
An upgrade patch introducing HDR support will soon be released for the PS4™ Pro console as well.
List of changes for Xbox One X:
- 2 new graphic modes: 4K Mode and Performance Mode
- support for 4K resolution
- HDR support
- enhanced shadows, ambient occlusion and texture filtering
- higher resolutions textures
- dynamic resolution scaling (from 1080p to 4K)
http://thewitcher.com/en/news/view/12581/
PS4 Pro will also get HDR but no word on PC.