[vlovich123]

I don't think you're being generous with the author's statement, especially since this is in the section within which he's describing chroma subsampling. The author is stating "We use 4 as a convention. why is that the convention? No one really knows". That seems accurate to me. Do you have a clearer answer? Your Wikipedia link doesn't provide any enlightment AFAICT, although maybe I missed explanation?

[dylan604]

Just before this section the author discusses how the image is broken down into blocks. This section is where the definition of the 4s could have come from, but they left out, for brevity's sake I'm assuming, how those blocks are shaped.

"Now, let's break it down the differences between 4:4:4; 4:2:2 and 4:2:0:

The number of pixels that share color is determined by what type of chroma subsampling it is. Each sample is defined by a block of 8 pixels. The first number refers to the size of the sample and its pattern, which is typically 4 pixels wide. The second number refers to how many pixels in the top row will receive color or chroma sampling. The third number shows how many pixels on the bottom row will receive chroma samples"[0]

The block sizes and sub-sampling methods are also why there are warnings issued when trying to scale an image when the dimensions are not divisible by the block sizes. If you try to scale to an odd number, then the sampling within the blocks is broken. If you scale to a number not divisible evenly by the largest block sizes requested, then you also get issues.

[0] https://blog.westpennwire.com/what-is-chroma-subsampling