While YouTube has supported 4K videos on the service since 2010, today that supports grows to encompass 4K live video streaming for both 360-degree videos and standard videos, the company announced in a blog post:
Supporting this new format will let creators and partners stream incredibly high-resolution video, and let viewers enjoy the clearest picture possible when watching a live stream on 4K-supported devices. The image quality is just mind-blowing on screens that support it, and in 360 degrees . . . the clarity can truly transport you.