I'd say 4K videos are already here. My Lumia 950, released late 2015, records in 4K by default, and the iPhone 6S, released around the same time, also supports

I'd say 4K videos are already here. My Lumia 950, released late 2015, records in 4K by default, and the iPhone 6S, released around the same time, also supports it. They aren't the norm, but I imagine YouTube sees a fair number of 4K uploads.