So we have a barbarian, cleric, and sorcerer. We need a rogue.
Disagree. Obviously you can do it with three strikers and a leader, but having a defender would do you more good than another striker, imo. (Unless you're thinking of skill checks, but even for that you don't
need a rouge.)
Having said that, I'm currently involved in a side-game where the party consists of three strikers and a leader (warlord, barbarian, monk and my ranger). It works, of course, but having a fighter or a paladin would help so much. I suppose, for a smaller party, the Paladin might make more sense because of the healing, and obviously if you anticipate fighting a lot of undead shit, the Paladin will wade through it. If it was me, though, I'd go with the Fighter simply because I'm biased towards it. I haven't played a warden or seen one in action, so I don't know how they play.
@Mutantllama: How did you build your sorcerer? I made a wild magic sorcerer one time. Absolute garbage class, imo, but conceptually very cool, which is what matters the most. I don't think I'd want to use a sorcerer in long campaign, but I might play one again in a stand-alone game. A striker who misses half the time and barley does any damage when he hits... Waded through minions with chaos bolt, though.
Disclaimer: Can't stress enough that I still think the sorcerer is awesome. The role playing is more important than the combat. Unfortunately my groups games have tended to be encounter heavy.