Having looked at some of the YouTube sample clips, 4k video presented me with a few issues: -
1. My CPU, GPU and drivers combination were not capable of sustaining full rate playback.*
2. One clip suffered corruption every few seconds.
3. Whilst I have plenty of constant, uncontended bandwidth, YouTube does not.**
4. I don't have a 4k display.
OK, the last one is pretty obvious. Mind you, you need HDMI1.4 to transport 4k video from your computer to your 4k screen. Even then, HDMI1.4 supports that resolution at a maximum 24fps (i.e., not really suitable for your Windows desktop).
I don't think 100Mbps is needed for 4k video at the consumer level. Current encoders can squeeze 1080p24Hz into, e.g. 5.1Mbps average over the length of a movie with very good quality. 4k video contains 4.2667 times as many pixels as 1080p. Very unscientifically, this means less than 22Mbps to transport a high quality 4k video stream.
Of course, consumers are happy with shitty quality digital video, as witnessed by the many cable-TV channels using low bitrate encoding which nobody seems to complain about (at least, not enough for cable companies to worry).
YouTube is not exactly cable-TV quality, so I'm sure they would get away with 5Mbps for 4k video.
*2.2GHz Core2Duo, Nvidia 8700M GT, latest drivers.
**I have 20Mbps internet, one of the 4k test videos constantly paused during playback due to buffer underruns.