If your group likes inter-party conflict (the literary kind, not the player-argument kind) then go with a character with a differing religious outlook. With a Paladin and Cleric in the party there's some opportunity there.
From a mechanics standpoint, I think you could go with:
1. Fighter or Barbarian. More beef is always good.
2. Ranger. Tons of skills, good synergy with Rogue, very useful detective abilities (spot, listen, Track).
3. Warlock. Fun rp opportunities with Paladin and Cleric, useful skills, very useful abilities that takes some of the pressure off the Sorcerer.
4. Party booster such as a bard, marshal, or dragon shaman.
A lot depends on what kind of cleric is being played. Caster cleric? Booster cleric? Melee cleric? Ranged cleric?
Given the choice between meat shield and warlock, I think this party could most use/you'd get the most fun out of a warlock. Especially if the cleric and/or sorcerer takes monster summoning spells to provide meat shields on demand.
-z
