Upgraded graphics card on my PC and now can run the 4k stream. Big difference!
I play Stadia with a Windows PC connected to a 1080p TV in my living room. (Or on my Chromebook if I'm in another room.) I like my TV just fine, and didn't really want to buy a 4K TV yet just to improve the look of Stadia. But, I only had integrated graphics, so couldn't force 4K. I got a GT 1030 card for $80, though, and I'm glad I did it. I changed my desktop size to 4k resolution, and now Stadia feeds me the 4k stream. It looks TONS better.
I know that lots of people wish the 1080 stream used more bandwidth to provide a better quality image. That would have been ideal, but anyway, I'm glad I can use the 4k stream now. I see a huge difference even though my TV isn't actually 4k.
Maybe when Android TV can run Stadia natively, I'll buy a new TV. In the meantime, this was a good compromise.