Well, how about a rouge? They're FAAAAAAAABulous!
Seriously...with that party makeup, I'd make sure SOMEONE in the party is some kind of alchemist. The ability to make firebombs, acid bombs, tanglefoot bags, etc. can multiply the effectiveness of your party. Acid solutions can get through locks almost as well as a rogue (except when magical locking is involved).
I personally don't think your party needs a pure meatshield warrior type, especially a barbarian. For survivability for a party of 4, flexibility and multiplication of power is key. Flexibility comes (IMHO) in the form of multiclassed PCs using Rangers, Rogues or Bards with some additional arcane spellcasting ability.
A high- Int ranger is almost as skilled as a rogue, and has much more fighting ability and overall survivability. I'm currently playing a multiclassed Rgr/arcane spellcaster who is almost as good a rogue as the party rogue, at least for things like opening locks, scouting, etc.
Also consider a specialist mage:
A specialist Diviner can do much of the info gathering rogues & scouts do, especially with levels of Ranger or Rogue thrown in.
A specialist Transmuter can buff the party like mad, including nifty spells like Bull's Strength and Spider Climb.
A specialist Illusionist has
styyyyyyyyle. They make decent infiltrators and con men, and with good spell use, they can really help control the field of combat to the party's advantage.