Yes, all games at launch support 4K. We designed Stadia to enable 4K/60 (with appropriate TV and bandwidth). We want all games to play 4K/60 but sometimes for artistic reasons a game is 4K/30 so Stadia always streams at 4K/60 via 2x encode.-- Phil Harrison (@MrPhilHarrison) October 9, 2019
All of this of course depends on available bandwidth. However, it is good to have it cleared up that 4K is a persistent option at launch. In other Stadia related news, Google hopes to alleviate concerns about latency with its cloud platform by aiming for something they call "negative latency".