Id-software is using virtual texturing. VT has (almost) constant texture memory cost, no matter how high texture resolution or how many different textures you have. With VT you can basically use all your assets in every single level, and VT only loads those texels that are currently visible. As there are fixed amount of pixels in the screen (~2M for 1080p, ~8M for 4K) there is a hard limit of texels you need to have loaded.
For example our Xbox 360 games (~720p) had a fixed size 48 MB VT texture atlas in memory, and that was used to store all loaded texture data (except for some special cases). Players could create levels with dozens of 4K textures visible simultaneously, but the VT system would never use more than 48 MB of memory. This works fine, as each new visible surface covers as many pixels as it adds, roughly maintaining a constant 1:1 visible texel ratio. Virtual texturing doesn't load texture data for hidden surfaces, so overdraw doesn't count.
With virtual texturing, a single 256 MB texture atlas should be enough (to hold all texture data) for a modern PBR pipeline at 4K. Additional texture data might however be needed depending on how you do your decaling and how you composite your materials at runtime. Id-sofware is nowadays using "dynamic" virtual texturing, meaning that they no longer just load baked data. VT pages are generated at runtime (similar to the RedLynx system). This is a great way to reduce the disk storage, and amortize decaling and material compositing, etc cost over multiple frames.
