I downloaded it with youtube-dl because I cannot stand the excessive adverts on Youtube these days. During the download it grabbed this video file:

  Input #0, matroska,webm, from 'New - Mars In 4K-ZEyAs3NWH4A.f313.webm.part':
  Metadata:
    encoder         : google/video-file
  Duration: 00:10:08.41, start: 0.000000, bitrate: 4436 kb/s
    Stream #0:0(eng): Video: vp9 (Profile 0), yuv420p(tv, bt709), 3840x2160, SAR 1:1 DAR 16:9, 29.97 fps, 29.97 tbr, 1k tbn, 1k tbc (default)

So yes it's 4K.