Europa Universalis 4 Wiki:Tiers
These are the 4 tiers of the EU4 countries with tier 1 with the highest quantity of flavor and priority and tier 4 with minimum flavour and priority. The tiers reflect developer priorities in designing the game; they are not an in-game mechanic.
High flavour and behaviour vital for gameplay.
Fairly high flavour and behaviour important for gameplay.
- Poland/The Commonwealth
Medium flavour and behaviour not essential for gameplay.
All nations not mentioned will be tier 4 nations and will expect flavour comparable to EU2 + EU3
Do note that Paradox might add flavor and behavior to other countries as they expand the game with free updates and DLC. The first expansion, Conquest of Paradise, adds new flavor and mechanics to Native American nations.