I'd be inclined to Wizard for the last as there isn't a lot of goon sweeping in the TOB-oriented party. Anything other than a summoning-focused guy will be good -- a summoner would stomp too much on the melee bailiwick of the other characters. An Archivist or a Cleric can do great b/c, well ... they can do all sorts of stuff, too. It probably depends on whether the TOB characters want to play bodyguard or not. If they don't, then the 4th should make a character who is as hardy as they are (totally doable). On the other hand, they might really enjoy guarding the squishy, and then he should focus on buffing and blasting and large-scale battlefield control.
Frankly, though, I tend to build characters based on a concept rather than a single class. I mean, what do these people want to play? I can think of lots of cool T4 pairings with the TOB classes: although I happen to find the tiers system silly and it unnecessary to gestalt TOB classes, but whatever. Crusader//Paladin, Swordsage//Sneak Attack Fighter, Warblade//Rogue with a Tiger Claw focus or even Warblade//Monk for a mini-smack build plus Warblad goodies, Warblade//Marshal or Knight for an ultimate battlefield commander, Swordsage//Warlock for a more supernatural angle ... the list goes on and on man.
Bring us concepts, and we can help optimize. This is all too abstract to really get a handle on.