As a result, I've left and joined 10 games for Act 2 Quest 1 Nightmare just now, and been placed into 2-man groups every single time. Since the automated matchmaking system is my only method for getting into multiplayer games, I have no way to over-ride this and I'm stuck playing 2-man Diablo 3, with an occasional 3-man run.
This is really frustrating. I want to play with 4 people. How is this acceptable?