"CSS4" is a bit of a misnomer. The CSS spec was modularised so all the various parts (like selectors) are versioned independently of each other. So there's no

"CSS4" is a bit of a misnomer. The CSS spec was modularised so all the various parts (like selectors) are versioned independently of each other. So there's no longer one version of CSS but a combination of various specs that each might be at totally different versions. Besides, no browser ever fully supported ALL of a particular version of a spec anyway.