Sorry if this has already been answered.
Typically, how easy will it be to upgrade a project using CI3 to CI4 (when it's released)?
It's not clear yet since they have not even got to the Alpha phase of ci 4.0, but we can already see that it won't be a simple process. CI 4.0 is a complete rebuild, and while there will be a lot of similarities, most of the code is completely refactored.
Codeigniter is simply one of the tools you need to learn to be a successful developer. Always add more tools to your coding arsenal!
And it is really exciting and I cannot wait. I stopped looking at the development branch to tease myself a bit :-)