There has never been a CSS4. There will never be a CSS4. CSS4 is not a thing that exists.
Flexbox ain't exactly new. So we'll put that and stuff of its era into a 'CSS4' bucket, and then put a lot of the newer stuff into a 'CSS5' bucket.
We can't just call everything after CSS3 'CSS4' because it's just... too much. I'm cool with it, let's do it.
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