Hi there ,I just received my stadia premier edition,
Checked my performance speed it's 215mbs yet my internet is too slow for 4k that requires 35mb
Sorry to hear, I would suggest though to walk through these TS steps in the Stadia help to try to fix your issue with playing in 4K, because there can be multiple reasons why you're unable to play in 4K.
If nothing helps, you can also try to contact the Stadia support as they maybe have further steps for you to try or they can also check if this is a possible bug.