On a side note, I'd think about the fact that you guys are also using magic items. let's say that you choose to have the king killed, thu saving the kingdom. You'd be revered as the new hero's, and become king/s:
the following happens depending on whether you choose to have 1 class fight at the end or all four:
One class: You defeat/kill the king, but as you cast away the magic items and revered as the new hero, thus becoming king, some form of sign initiates leading the player to believe he's going insane thus doing two things. The first is to create an opening to a new game, an end-game clinger that leads to people wanting more, creating a community that'll help you to your second game. The other thing is to really, well, create a meaningful story. Your the persecuted villain who defeats the false hero to be revered as a hero only to change into a villain. If that isn't an interesting plot, I don't know what is.
All classes: The effect would be much the same, the only difference is that you divide the kingdom into four and as you havea gathering you notice one of them showing a sign of insanity. While it's a nice ending, its kind of a cliche beginning if there is a next game, three hero's fighting a villain While with the other one, you can do really anything with the part two story. So I would recommend only having one class.