It’s indeed not really written anywhere because likely to evolve but basically :
3.7.x : the major feature is a new (or rework of the) permission system + handling of user groups. You can get an overview in the testing changelog. It’s currently in testing since a while because it’s pretty touchy and we have some nasty edgecases which are not straightfoward to test/fix.
3.8.x : Modulo a few small details, it’s basically already complete and will contain a new diagnosis system to help setting up your server (e.g. DNS records, port forwarding, and various other things) + a classification of apps by categories. Maybe other things, but I don’t have the full changelog in mind (+ maybe some other stuff will be merged by then).
4.0.x : It will be the support for Buster which is still in development / early alpha stage but coming together. With this, we’ll ship a migration procedure to people running 3.8 / Stretch to upgrade as smoothly as possible to 4.0 / Buster
4.1.x : ???