In Descent, the balance comes from the number of monsters changing based on the number of heroes. Based on previews of IA, it seems like this is no longer true and instead fewer heroes will get more actions.
This seems alright, except for the fact that the Imperial player can win by knocking down all heroes. Wouldn't that be way easier against 2 heroes as opposed to 4, regardless of how many actions they have? It's still 1/2 the total HP to knock off.