Video players usually keep a forward buffer of a few minutes of video
You still need good bandwidth to ensure the video starts playing right away, plus enough bandwidth to be able to buffer ahead while lots of people are using the service at the same time. I’ve seen some video sites that need to buffer for a looooong time before the video starts playing.
A few minutes of video at 4K takes a noticeable amount of space, so I don’t think video sites would buffer that far ahead for 4K.