migrating to CI4

I am looking seriously at migrating from CI 3.x to CI 4.x. However, I have a complex website with several thousand of lines of code and dozens of libraries, controllers and models. The thought of rewriting is daunting and if necessary to spend months on a conversion, begs the question why stick with CI?

 Is it as simple as changing how controllers, libraries and models are defined to fit into the new concept plus messing around with a few config files?

 My long term concern is staying current with PHP into the future and I have no idea how compatible CI 3.x and 4 are with PHP8. I suspect this is not an issue but it is always worrisome.

 Another question is: does CI4 have more robust security features?