I think that Flexbox is really easy to understand, you can try and play with it on flexboxfroggy.com it will help you understand how the single property play together with real-time feedback and lots of fun!
Correction: if I do not get wrong there will no be CSS4 since the development on this project has been setted a per-module feature, like HTML5, so it is going to be pointless to add a version number. We see how this will play out since will be really difficult to understand how we can talk to each other :D