The number 444 refers to the luminance (light) and chrominance (color) compression (brightness and color) (see Figure A4.1).
FIGURE A4.1
Chroma-luminance bandwidth illustration, where a 4:4:4 has no subsampling (each pixel contains the full luminance and chrominance value). In a 4:2:2 subsampling, half the pixels contain full luminance and chrominance value, while the remaining half contain luminance.
(http://lea.hamradio.si/~s51kq/V-BAS.HTM.)