Simple Answer: Only what's in the current "core" folder right now.
Then users could drop in Database, Helpers, Libraries, Language folders as needed to make it more.
A smarter PSR4 style autoloader would probably support this pretty easy (just a assumption of course)
Documentation wise probably 3 or 4 pages explain what it doesn't include but since it would actually be 100% the CodeIgniter "core" those topics in the main Documentation would be valid. Most importantly is a separate download to not confuse people.
Every CI program uses Core and that is probably where the team will spend most of there CI4 development time anyway.
The Core could also be a early "test bed" approach to CI4. Sure it won't have Calendar, Email, etc... and all the documentation but, it should have enough for adventurous dev's to throw some stuff at it to test speed, loading resources, resources usage, etc... and to see how nicely it can work with other PHP libraries (not CodeIgniter).
DMyers
