Be warned there is no such thing as CSS4, but it's OK because there wasn't really any such thing as CSS3 either. Since the completion of the CSS Level 2 specification everything has been broken down into modules, 'CSS3' isn't supposed to mean 'the third version of CSS' but 'modular CSS specifications'. Although the initial 'CSS3' stuff was all based on parts of the CSS2 specification (hence they are 'Level 3'), a lot of the new things (eg. Flexbox, Grid Layout and Regions) are Level 1.
Although I suspect the CSS WG will have a hard time convincing the rest of the world about this, so we'll be seeing plenty of 'CSS4' as a buzzword in the future, but it will be mostly meaningless in the same way that the buzzword 'HTML5' currently is. People will just use it to mean 'new stuff we can do in browsers'. The important thing isn't which level an individual specification has reached, it's whether browser support among your audience means that you can make use of the features in it.