1) Codeigniter 4.1.8 is a security update:
XSS Vulnerability in the API\ResponseTrait was fixed.
2) Codeigniter 4.1.7 brings the following main update:
FILTER_SANITIZE_STRING - replaced
3) Codeigniter 4.1.6 also brings some security updates, such as:
Deserialization of Untrusted Data found in the old() function was fixed.
fix: Validation cannot handle array item
As a conclusion, for security reasons you should always update to the last version of the framework that you're using, even if it's Codeigniter, Laravel, Symfony or others.