The 15 Mbps is TCP bandwidth, and the 100 Mbps is physical layer bitrate. They are not comparable. There are overheads and latency and packet loss can reduce how much physical bitrate TCP can use. So the 4k stream may or may not do great in 100mpbs.
Also, how about a second 4k screen ? Multiple TVs are the norm in American homes. How about multiple tablets or mobile devices consuming HD streams ?
It seems current access link infrastructure in the US cannot support current TV viewing habits, if all the TVs were switched to 4k digital streams.
> The 15 Mbps is TCP bandwidth, and the 100 Mbps is physical layer bitrate. They are not comparable.
In practice TCP gets very close to the link rate unless you have latency much higher than what you typically have on cable/fiber. Fast.com (which "performs a series of downloads from Netflix servers" according to Netflix) benchmarks at 155 mbps on my 150/150 connection. (Cable/fiber systems are usually provisioned at a higher link rate than what is marketed: http://www.dslreports.com/faq/15643).
Netflix won't enable 4K on a 15Mbit connection. Perhaps 50, definitely not 15. Remember they have to allow some headroom and for the possibility of more than one concurrent stream to the same subscriber.
Netflix 4K streaming is only about 15mbps (and looks pretty great), so 100mbps is definitely more than enough.