Upgrade CI3 to CI4, ORM, other...

Hello,

It's my first post. So, hello everyone!

1) I am working with some large, complicated app. I am going to do some refactoring and maybe it's good time for upgrade CI. What are biggest differences between 3 and 4?

2) While refactoring I am going to use ORM and some DDD to make my work easier. (Yes, it's not perfect solution but for me it is the best.) Could You recommend me some ORM for CI 4 (and 3?). I read about GasORM but it's compatible with CI2. I can use Doctrine/Elqoquent/CakephpORM but I prefer more Codeigniter-way library (if it possible).

3) Is Codeigniter4 stable? There are no bugs (within reason) f.e. at sessions?

4) There is some library (codeigniter-way) for automating crud (scaffolding)?

5) I always wonder, why CI uses system directory instead of vendor?

I am full of admiration for you that you develop and is getting better! Thanks for all