How to upgrade from CI3 to CI4

It is very hard to understand the upgrading documentation anyone know how to do upgrade from CI3 to CI4

I could not (yet) find a good description of what and how, so I guess you need to figure out - re-learn CI if you will.
There are many breaking changes. You cannot just simply upgrade the framework and expect your app to work.
You can reuse your codebase, but with heavy refactoring.

(off topic, but the journey into CI4 changed my coding style for the better)