VUlnerability

CI4's Security Advisories are there.
See https://github.com/codeigniter4/CodeIgni...advisories

> Must we update every new version?

No, if there are not known vulnerabilities.
Now v4.3.5 or later does not have known vulnerabilities.

But updating is recommended.

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper