@Aitha Samples
In these samples I only upscaled all the face maps, I didn't touch the lips, in the stock 4k renders I think I accidentally added a normal map for the lips which seems to be missing from the 16k renders.
The resolution doesn't matter from longer view distances, might even make it look worse, but it's meant for closeups.
But like I said in my original post, it's mostly useable only on models with lower res textures, like 1-2k, instead of upscaling already good looking textures like 4-8k 🙂