I don't believe YouTube uses 50mbs for 4K. I know they don't use HEVC, but that requires half that bitrate, even for a poor implementation like x265.
YT uses AV1 and VP9 for 4K, and even shitty VP9 tops out at less than 30mbs in the following example, with 4K options in the ladder at a lower bitrate than this (down to 12mbs for AV1). This is the for the "Peru" 8K60 video, with anything below 1080 in the ladder removed:
% yt-dlp -F 1La4QzGeaaQ
[youtube] Extracting URL: 1La4QzGeaaQ
[youtube] 1La4QzGeaaQ: Downloading webpage
[youtube] 1La4QzGeaaQ: Downloading ios player API JSON
[youtube] 1La4QzGeaaQ: Downloading android player API JSON
WARNING: [youtube] Skipping player responses from android clients (got player responses for video "aQvGIIdgFDM" instead of "1La4QzGeaaQ")
[youtube] 1La4QzGeaaQ: Downloading m3u8 information
[info] Available formats for 1La4QzGeaaQ:
ID EXT RESOLUTION FPS HDR CH | FILESIZE TBR PROTO | VCODEC VBR ACODEC ABR ASR MORE INFO
...
312 mp4 1920x1080 60 | ~ 299.37MiB 7256k m3u8 | avc1.64002A 7256k video only
299 mp4 1920x1080 60 | 205.09MiB 5094k https | avc1.64002A 5094k video only 1080p60, mp4_dash
617 mp4 1920x1080 60 | ~ 250.64MiB 6075k m3u8 | vp09.00.41.08 6075k video only
303 webm 1920x1080 60 | 163.57MiB 4063k https | vp09.00.41.08 4063k video only 1080p60, webm_dash
399 mp4 1920x1080 60 10 | 106.06MiB 2635k https | av01.0.09M.10 2635k video only 1080p60 HDR, mp4_dash
699 mp4 1920x1080 60 10 | 201.11MiB 4996k https | av01.0.09M.10 4996k video only 1080p60 HDR, mp4_dash
636 mp4 1920x1080 60 10 | ~ 305.96MiB 7415k m3u8 | vp09.02.41.10 7415k video only
335 webm 1920x1080 60 10 | 271.48MiB 6744k https | vp09.02.41.10 6744k video only 1080p60 HDR, webm_dash
623 mp4 2560x1440 60 | ~ 581.83MiB 14102k m3u8 | vp09.00.50.08 14102k video only
308 webm 2560x1440 60 | 451.93MiB 11226k https | vp09.00.50.08 11226k video only 1440p60, webm_dash
400 mp4 2560x1440 60 10 | 244.91MiB 6083k https | av01.0.12M.10 6083k video only 1440p60 HDR, mp4_dash
700 mp4 2560x1440 60 10 | 555.74MiB 13805k https | av01.0.12M.10 13805k video only 1440p60 HDR, mp4_dash
639 mp4 2560x1440 60 10 | ~ 730.05MiB 17694k m3u8 | vp09.02.50.10 17694k video only
336 webm 2560x1440 60 10 | 649.63MiB 16137k https | vp09.02.50.10 16137k video only 1440p60 HDR, webm_dash
628 mp4 3840x2160 60 | ~ 1.15GiB 28477k m3u8 | vp09.00.51.08 28477k video only
315 webm 3840x2160 60 | 999.89MiB 24837k https | vp09.00.51.08 24837k video only 2160p60, webm_dash
401 mp4 3840x2160 60 10 | 484.74MiB 12041k https | av01.0.13M.10 12041k video only 2160p60 HDR, mp4_dash
701 mp4 3840x2160 60 10 | 994.25MiB 24697k https | av01.0.13M.10 24697k video only 2160p60 HDR, mp4_dash
642 mp4 3840x2160 60 10 | ~ 1.28GiB 31765k m3u8 | vp09.02.51.10 31765k video only
337 webm 3840x2160 60 10 | 1.13GiB 28868k https | vp09.02.51.10 28868k video only 2160p60 HDR, webm_dash
571 mp4 7680x4320 60 10 | 1021.79MiB 25381k https | av01.0.17M.10 25381k video only 4320p60 HDR, mp4_dash
702 mp4 7680x4320 60 10 | 1.95GiB 49706k https | av01.0.17M.10 49706k video only 4320p60 HDR, mp4_dash