Louis Lazaris, reacting to the idea of CSS4:
The reason “CSS3” worked is because it was real. It was the successor to “CSS2.1”. Everything after CSS2.1 was considered to be under the umbrella of “CSS3”.
The gist is that CSS4 isn't real, so won't work, and we don't need it anyway. Perhaps I overestimate the power of marketing, but I'd wager Louis underestimates it a bit. When an idea takes hold, basis in truth or not, it has tremendous power and money follows. See: politics and pet rocks.
Amelia Bellamy-Royds pointed out that there is a real thing we could point to, and those are "CSS Snapshots" that the working group produces that could be marketed as yearly versions. Like we have ES2019, we could have CSS2020.