[Olorin]

You might even consider another c/m. It's hard to turn Aerie into a tank because of her stats, but a protagonist c/m can be quite effective. You won't have much ApR, so don't expect to melee enemies to death. Use the huge amount of spellcasting power that your party would have to devastate them. When I played one, I used my c/m as the tank/bait, and when all the enemies converged on him, he fired off a sunfire, while Jan, Aerie, and Nalia dropped fireballs on him. You can try to be really tricky and time the fireballs to hit while you are still immuned to fire from the sunfire spell, or just use fire protection spells/items.

Otherwise, I'd go for the cleric/ranger or the figther/mage. This will give you the ApR's from a warrior class and either stone or ironskins and other buffing magic. The cleric/ranger would have the weapon restrictions, but with only four in your party you can't use all the really good weapons anyway.

Jan should be all the thief you need. Most stealth can be accomplished with the use of invis spells/items/potions anyway.