The 2 main characters are involved in the main plot of the game. Henchmen have no quests, they are just mercenaries. Companions do have quests associated with them, and comments to make on various situations; you can recruit companions fairly shortly after getting to Cyseal, if you know where to go, without opening the hall of heroes.
Having 4 characters at character creation would mean either 4 main characters (more than doubling the amount of dialogue needed for them, and making quests and QA more complicated), or 2 main characters and 2 custom henchmen (which would make the companions, and all the work put into their quests, kind of pointless).