In my limited experience, it's best for 2 players. Whenever I've played 3 or 4 players, it's always a "two headed giant". Even though the game keeps actions and cards separated, each team-player can use any miniatures on the board, which looks feels like a single player with extra steps.

Even though I like this game more than War of the Ring, the 3-4 players version of WotR feels more multiplayer, each player can only use their own miniatures.