What?! Zerash channels negative energy? Isn't that, well, er... evil?
Anyhow, I think what the party needs in a fourth (This is assuming Durgen leaves, which I hope is not the case) are these qualities:
stealth: we could use a scout.
woodlore: adventurers spend a lot of time in the woods, and it helps to be able to follow tracks, ect.
keen senses: I think everyone in our group except Durg was pretty pitiful at listening and spotting. It'd be nice to avoid the occasional ambush.
healing ability: This is always nice, and it'll save a bunch of coin being spent on wands and scrolls.
attack-soaking ability: basically just stepping in and taking some hits, or at least attacks. Walf is going to be hurting if he's the only one ever standing next to baddies. Zerash can step up, and he's hard to hit, but after his bad luck last time, he'll be hard to convince until it's too late, and it's more difficult to heal when you're in AoO territory.
Charm: Currently, our most well spoken party member is a lizard. Still, we're probably not going to get hacked to pieces for lack of eloquence. Probably...
The following classes do pretty well in these categories:
Druid: pretty good healing, keen senses and possibly tracking. Plus, pretty soon they can turn into animals and stuff, and that's always useful. Only problem is that they tend to be horrible bores, and you end up having to sit through endless lectures on fire safety and they're always nursing sick animals.
Ranger: Pretty good in just about every quality, except healing, where they're pretty lousy unless they have a wand, and even then it's not much.
Rogue: A rogue'd be more useful if we were going to be doing a lot more dungeoneering. Which we may do. Plus, rogues are fun to have around.
War Cleric: Pretty good, though these types tend to get you into trouble with their enthusiasm for fights, and lots of them. Plus, still no keen senses.
Gnomish Healer: Walf could keep him in his backpack.
Just some ideas. Of course, he/she should play whatever she likes, but I think these will be the most useful to us.