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.
I recently was contacted by someone needing help on their network who explained to me that the 'other guy they were talking to' promised to make their site CSS4 compatible. Then they asked me when CSS4 was going to be available. So, here is your answer the next time a swindler tries to take your money while trying to promise you the world.
Actually, promising you the world would be better because the world actually exists. CSS4 doesn't. Just fair warning.