Job: migrate CodeIgniter 3 to CodeIgniter 4.
CI3 and CI4 are two completely different architectures. No magic update, no automatic script. You have to do it by hand.
I created a new CI4 project from scratch and migrated the features one by one. The method: copy, test, fix, start again. Some modules had disappeared in CI4, so I found alternatives. Others just needed an update, and that's done.