help me decide

In short: Codeigniter is fast and easy to learn. And it has a lot of nice people here at the forum.

I've read (almost everywhere) that CI 3 is easy to learn. Does this apply to CI 4 as well?

As a framework, CodeIgniter is relatively lightweight, meaning that you will need to write code or use third-party code for some commonly needed, more complex tasks, i.e. authentication. In other words, it won't solve every problem for you out of the box. IMO, that's a good thing because you have lots of latitude for problem-solving.

I would not start with version 3. The new, soon to be released, version 4 utilizes current best-practices. It's a thoroughly modern rewrite of v3. While v4 will likely have some growing pains it will be very stable for a first release and I expect fixes will be quickly available for any problems as they are discovered.

I have been working with the code-base as development has progressed and have found it impressively stable and enjoyable to use.

In the long-run, version 4 is the way to go. If you start with v3 you will have to learn new (and current best-practice) syntax when you move up to v3. As a learner, it would be wise to learn the current state-of-the-art.

Learning either one is equally easy - and challenging.