What will happen to CI3 once CI4 is released to production?

I understand there is not a brilliantly easy upgrade path from CI3 to CI4.

What will happen to CI3 - will it continue to be developed or will an End of Life date be declared?

I have a major project due to start in the new year and this will help my decision on which CI version to use - I'm currently leaning towards CI4 even though it is still in pre-release.