[keithwinstein]

Poynton has a pretty plausible-sounding explanation here (https://poynton.ca/PDFs/Chroma_subsampling_notation.pdf):

"The commonly used leading digit of 4 is a historical reference to a sample rate roughly four times the NTSC or PAL color subcarrier frequency; the notation originated when subcarrier-locked sampling was under discussion for component video. Upon the adoption of component video sampling at 13.5 MHz, the first digit came to specify luma sample rate relative to 3 3⁄8 MHz. HDTV was once supposed to be described as 22:11:11! Since then, the leading digit has – thank-fully – come to be relative to the sample rate in use. Until recently, the initial digit was always 4, since all chroma ratios have been powers of two – 4, 2, or 1. However, 3:1:1 subsampling has been commercialized in an HDTV production system (Sony’s HDCAM), so 3 may now appear as the leading digit. By convention, a leading digit of 2 is never used."

And here is lots of detailed history: https://tech.ebu.ch/docs/techreview/trev_304-rec601_wood.pdf , including a lot of debate in the late 70's about "three-times sub-carrier (3fsc) versus four-times sub-carrier (4fsc) sampling." The victory for team "4" is, I think, why that's the leading digit, even though they ended up compromising on not-quite-4 in the end.