Form verification callback function in Ci4

Hello,

I have been using Ci3 for 3 years. I have developed many projects. I created a code pool for myself. Learning Ci3 is very easy. You can find many examples on the Internet. But Ci4 is not like that. I had a hard time learning. The documents are unfortunately not open enough. There is not enough sample code on the Internet. Obviously my enthusiasm was very broken.

For example, form validation in Ci3 is as follows:

Code:

$this->form_validation->set_rules('password', 'password', 'required|callback_check_database');

How can I do this with Ci4? Could you please explain it simply by writing code?