Yes, You heard it right Stranger things 4 is announced and the teaser for it has been released on 15th feb
However no any conformed dates were announced for the series to release
It is expected to release in 2020 anytime.
On the teaser. We can see hopper who everyone thought dead is working for the Russians. (Probably undercover but who knows)
To answer all your questions you have to wait for the Season to release on Netflix Soon