Clarification: 4 players, max party of 6 (2 NPCs). That is the suggestion.
And the point is that every encounter in the game right now works with party of 5 or 6 if you use proper stats and abilities. The reason encounters are severely nerfed is because they tried building it for party of 4 or less max. Thus, they severely hinder the game and limit encounters to fit the 4 or less party max mold.
The game would benefit therefore from a party size 6 so they could actually legitimately have players face what they designed the game for them to face instead of stripping monsters of abilities and behaviors that make them what they are.
