4k is not the dominant resolution for some media like twitch streams. There are also lots of people who still don't have Internet fast enough to stream native 4k. It could help with older YouTube videos, as well.
I need to look for the statistics again but most media is still served at 1080p. If you look at the most popular YouTubers however the skew is towards 4k because Google wants to tout that number, however the bitrate is comparable to 20mbit 1080p