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