I regularly get people asking about when "CSS4" will be available.
I'd like to clear things up with something unequivocal:
THERE IS NO SUCH THING AS CSS4
There has never been a CSS4. There will never be a CSS4. CSS4 is not a thing that exists.
The term "CSS3" refers to everything published after CSS 2.1.
CSS is on its last version as a language as a whole, so it would be appropriate to just drop the number entirely and refer to everything from now on as just "CSS".
"But", you might object, "I saw things like CSS4 Images, or Selectors 4! Aren't they part of CSS4?".
No. As I just said, there is no CSS4. ^_^
While trying to finish CSS 2.1, we (the CSSWG) realized that big monolithic "versions" weren't any good. They were difficult to maintain, and slow to develop.
Instead, we decided to split up the CSS language into a bunch of independent modules. Each module can level up independently, and contains only a smallish set of features, so it's harder for a large set of features to be slowed down by a single stubborn feature.
Some of our modules start out at level 3, if they extend something from CSS2.1. Others start out at level 1, if they're something new (for example, Flexbox). However, the level that a module is at has no correlation with what version of CSS it's in. They're all CSS3 (or just CSS), regardless of what level they're at.
Our URLs don't help the matter much, of course. We use URLs with things like
css4-backgrounds in them, which totally sounds like CSS4. On behalf of the CSSWG, I apologize for the confusion. All that means is that it's CSS Background & Borders Level 4, which is implicitly part of CSS3 (or just CSS).
So, now you know. There's no such thing as CSS4. There is only CSS, and each module can level up independently. Pass it on!