RED4ext is currently only a plugin manager for plugins that extends REDengine 4. It takes care of loading the plugins for the specified game's version.
RED4ext.SDK is a library that can be used to extend REDengine 4. The library provide the following features:
Interacting with the game's scripting VM (e.g. calling functions or accessing class properties);
Creating new native classes or functions;
Reversed game's structures;
Where to find the documentation
You can browse Red4Ext via Rayshader's excellent web tool NativeDB, where all documentation will be gathered and kept up-to-date by volunteers.
If you want to get involved, check out the github repository or this thread on the REDmodding Discord.