Just after he says: "4:2:0 is the most popular case. Four luma samples per one chroma" so he does understand and write what it means. That does not explain why

Just after he says: "4:2:0 is the most popular case. Four luma samples per one chroma" so he does understand and write what it means. That does not explain why the value is 4. I assume it is because with 4 you have 3 different subsampling ratios (if you want to keep factors of two, which you typically want to keep algorithms simple)