Yes, J4 documentation is hard to come by. There's some good news though. Firstly, although J4 is quite different in some respects, there are also many similarities. There's the beginnings of a guide at
Don't be put off by the first page, go to the second and you'll start seeing real information. That'll give you information about the differences, and then you'll be able to get more value out of the J3 docs too.
Other than that, ask specific questions either here or the J! community forum and hopefully you'll be able to get some useful feedback.