I see alot of threads with fragments of information.

Trying to get the whole picture.

I'm dealing with a heavily customized 2.x Limesurvey installation, with hacked files all over the place, additional controllers, models, helpers scattered through out the core directory structure.

I want to move every addition/hack out of core, and into a plugin, or a module, or an extension, or "something", so that upgrading is only a nightmare this last time.

Have development best practices been documented for 4.x yet, that is how to extend it?

There's so many concepts here, plugin, module, extension, helper etc.. When is the "right" time to use which?

I see pages like this:
github.com/LimeSurvey/LimeSurvey/tree/de...admin/globalsettings

Where it describes that it will (or is?) possible to develop Yii modules to integrate with LS
Will this allow addition of controllers for non-admin routes?
How about models and helpers?

Is this fully baked in the most recent version of 4.x?

4.x is and has been marked as unstable for some time. Is there any idea when 4.x would be considered stable and ready for production?