From Eurogamer: “Doom Eternal is not true 4K on Google Stadia, developer id Software has confirmed.
Rather, the blistering first-person shooter runs at 1080p and 60 frames per second on HD displays, and up-samples to 2160p from 1800p at 60 frames per second on 4K displays.”
Source: N4G PC No, Doom Eternal is not true 4K on Google Stadia