I'm able to get it to play at 4K in Firefox and IE11 (desktop version) on Windows 8.1. Don't have a W10 version handy to try with Edge. At least it says its 4K on the setting, I only have a 1080P monitor for output but it seems to be the 4K stream and it plays.
What platform are you on? Google sponsor Chrome and own YouTube, and it wouldn't be the first time they've played favourites, but it seems to be working for me.