Enabling Ultra Textures
To successfully enable the Ultra texture pack, you must first download the High Resolution Texture Pack DLC from Steam.
Navigate to the game page in your library, click on "Find more DLC in Store", and select and download the texture pack (10 GB). This will put the file uhdmip.arch06 into your game's folder. This file contains all the Ultra textures.
If you don't download the texture pack, the "High" and "Ultra" settings will look exactly the same, even though VRAM usage is increased as if you had the Ultra Texture pack installed.
Ultra Texture Comparison
The following screenshots are 1:1 crops of 4K screenshots using the available texture settings.
I picked the area with the most visible differences. The full-screen screenshots further below show how small the differences actually are (between high and ultra textures).
Low
Medium
High
Ultra
For reference, here are the original loss-less-compressed 4K screenshots:
Low
Medium
High
Ultra