How to make my app an CI4 from CI3?

You need to code again your projects. However, the basic understanding is similar.

• But as always, learn the models. This time model files are a bit different.  Model on CI User Guide
  

• Extra for Models: Query Builders
  

• There are some differences with Controller as well: You can send header and footer on Controller: View Part on User Guide
  

• You can even save file or delete it in controller: read model page again: Model on CI user Guide
  

• Also Redirect line within your controller is a bit different
  
  return redirect()->to($_SERVER['HTTP_REFERER']); // It  an example of redirect to the same page. You can even send a message: Redirect Page On User Guide
  

• Be careful with the namespaces.