The idea behind basing CSS4 around a series of modules was aimed at making it easier (and ultimately quicker) to update each standard; it does mean that modules will be in a state of flux, at least for the time being!
This said, there are some clear styles we can recreate using current CSS3 styles—one such example is CSS4 Selectors. Even though it is still in draft at the time of writing, a PostCSS plugin is available in the form of postcss-selector-not
(from https://github.com/postcss/postcss-selector-not). A sister style that is also available as a PostCSS plugin is postcss-selector-matches
(available from https://github.com/postcss/postcss-selector-matches)—these two are intended to replicate the :not
negation and :matches
pseudo-selectors that are coming as part of the new CSS4 standard.
Note
For more information on individual CSS4 selectors, take a look at the full list available at http://css4-selectors.com/selectors/. This will also give you an indication...