That 15Mb/s figure for 4K is out of date by a couple of years. They previously targeted a fixed average bitrate of 15.6Mb/s. They now target a quality level, as scored by VMAF. This makes their average bitrate for 4K variable, but they say it has an upper bound of about 8Mb/s. See https://netflixtechblog.com/optimized-shot-based-encodes-for...