First, welcome to Tikiwiki Community!
After that, you are right, documentation for the new behavior for categories in Tiki4 is not up to date, and migght be confusing for new admins.
In short, you need to grant global permissions first for your group to create new pages. (through "Admin > groups > click on the key icons to grant global permissions for that group).
However, remember that:
- first rule object permissios, if they exist
- then category permissions are applied (if no single object permission is defined for that object), in case they exist for that object
- then global permissions rule, in case no object nor category permissions are defined.
It's pretty easy to start granting permissions at a global level, and then, restricting access to specific objects in a object by object basis. That should work for you to learn how to handle tiki permission system.
Once you master that, you can play with category permission in the middle (instead of object permissions, for instance; remember that object permissions make category and global permissions ignored for such feature)
Cheers, and welcome to join the effort to improve the documentation as you learn how to master permissions (or any underdocumented feature that you master).