So since the beginning of RPGs there's been a question - how many players is best? Some people have wishy-washy answers that it depends. It does not. Four players is best. While there can be an argument made for three, four is demonstrably similar and adds a completely new voice. At five and beyond, there will be a player at the table that wallflowers, either on purpose or by sheer inability to present themselves to the group because of the group. There is some semi-science behind this. Pseudoscience really because I can't be bothered to investigate conversational dynamics beyond my own life experiences.
If you're ever at a party, it's fairly impossible to participate in conversations with greater than four participants, often a fifth just ends up listening. This is fine in RPGs because your fifth, the GM, should mostly be listening and when he talks everybody should shut up and pay attention. RPGs are in the same vein. Get 10 people at a table and 2 distinct conversations (at least) will be going on. This is not conducive to roleplaying because the GM can only pay attention to one conversation, and therefore only one conversation is important at any given time, and anyone involved in the other conversation is wasting their breath and attention on something without value to the game session.
And yes, roleplaying in and of itself is not a reward in a game session unless the GM is paying attention to it. For the world to react properly the GM's attention must be on a player talking.
Why not three? Three players sometimes hit a snag and go for periods without talking, or sometimes two gang up on one when a decision is called for. It's simply more likely there will be less stress on the player's side with four players bouncing around. This is actually baked in to D&D itself, as (after OD&D) there are four core classes and therefore four core roles in the game. The entire game screams for four players, and nearly always has.
