CSS1 or CSS Level 1 was first proposed in 1994 by Håkon Wium Lie. It was later released end of 1996. It only had roughly 50 basic properties. It maily focussed on styling text on a web page.
CSS Level 2 saw more advanced properties allowing for position of page layouts and better styling in 1998.
CSS2.1 mainly focused on fixing errors and removing some of the CSS2 features that either didn't work or were poorly supported. The back and forth drafts and recommendations took almost 9 years to be officially published back in 2011.
CSS3 saw even more advanced features such as animations being added. The main difference between CSS2.1 and CSS3 is that the latter is split into different modules. Each module has different levels in its own phase of development or review. The main reason for this is that CSS2.1 took too long to be approved and published. So you could have some modules at level 1, never to be changed, others at level 2, and some modules even at level 4.
What is CSS4? There seems to be quite some hype around this new version. There are many discussions on when is it coming out, what is it, what features does it all include, and so on. Well, the shocking part of it all is that's not the case at all. CSS4 is not even a thing, nor will it be. It's not a new language nor a whole new version of CSS. It is simply just an online clickbait sensation.
CSS is one of my favourite languages to use for building websites. I love its flexibility of changing absolutely anything. Although one can appreciate new CSS feautres evolving and changing over time, don't get caught up in the online hype of CSS4. It won't be coming out and will never be released.
To see the full progress of CSS, go to: https://www.w3.org/Style/CSS/current-work