naming of yuv 4:4:4, 4:2:2

The numbers in YUV 4:4:4, YUV 4:2:2, and YUV 4:2:0 represent the chroma subsampling ratio, which describes how color information is stored relative to brightness (luma) information.

Meaning of the Three Numbers (J: a:b)

The standard notation J: a:b represents how chroma (color) samples are stored in relation to luma (brightness) samples in a 2×2 block of pixels:

J → The width of the reference block (usually 4, meaning a 4-pixel-wide sampling area).
a → The number of chroma (Cb, Cr) samples horizontally in the first row.
b → The number of chroma samples in the second row (if any).

ref

The first number (in this case 4), refers to the size of the sample. The two following numbers both refer to chroma. They are both relative to the first number and define the horizontal and vertical sampling respectively.

A signal with chroma 4:4:4 has no compression (so it is not subsampled) and transports both luminance and color data entirely. In a four by two array of pixels, 4:2:2 has half the chroma of 4:4:4, and 4:2:0 has a quarter of the color information available. The 4:2:2 signal will have half the sampling rate horizontally, but will maintain full sampling vertically. 4:2:0, on the other hand, will only sample colors out of half the pixels on the first row and ignores the second row of the sample completely.