Depending on the device you are watching a video from Youtube on, the service will send you a version of the video that fits your current device’s screen resolution, and also takes potential network limits into account. So if a video is available in 4k resolution, and if you have a 4k device, and the pipe between Youtube and the device is not the limit, what will the data-transfer rate be?
Almost by accident, I recently tested this ‘limitless’ scenario with this video on Youtube about Hetzner data centers. The play time is 23 minutes 34 seconds and if you stream it in 4k resolution, about 3 GB of data are transferred in that time. That’s an average data rate of 16 Mbit/s. Wow!