Converting a CI3 Project to CI4.5.1

Sorry, are you too lazy for documentation?   There are instructions for updating. I can't guarantee that everything is described, it's a big step.
https://codeigniter4.github.io/userguide...#libraries

I studied the CI3 documentation myself, fixed some scripts. Then I switched to CI 4. There were practically no problems with the migration.

You need to change your tactics. Not just copy the code, but reproduce the logic of the application.
For example, "load the model, output the model data" and not "create a variable to Copy/Paste replacing $load"

Simple CI 4 project for beginners codeigniter-expenses ( topic )