migrate big project from 3.1.X to 4.X

I'm afraid it will take hours to actually analyze all the links you have provided for someone to give any detail description how to port something like that. If you are not up to it yourself, you need to hire a developer I'm afraid as CI4 are a complete rewrite from the ground up, and it's breaking changes everywhere. Nothing can be modified to be able to work, you need to re-write it.

https://codeigniter.com/user_guide/insta...e_4xx.html

If you want to use "the latest and greatest" of CodeIgniter, it's version 4. Or you can of course move to another framework, but we would be sad to see you leave. Personally I haven't read any official statement that CI 3 are not compatible with PHP 7.4.

There are only one note about a PHP 7.4 fix in the not released CI 3.1.12
https://github.com/bcit-ci/CodeIgniter/b...ngelog.rst

And here are the changelog for the upcoming CI 3.2.0
https://github.com/bcit-ci/CodeIgniter/b...ngelog.rst

So CI3 are not dead.