CodeIgniter 4.0 is here!

It is stable enough that the dev team thought it should be released. It's not perfect, but it is stable.
Most of the current issues have more to do with installation problems than with overall reliability.

It's certainly more stable and secure than CodeIgniter 2.

There are a lot of changes in how things are done compared to the previous versions. There is a lot to learn, but not as much as you'll need to learn to be at home with Laravel. Especially true if you have no prior experience with Laravel. Prior CodeIgniter experience will be helpful when getting into CodeIgniter 4.