A Druid would make a good choice, I'd have to agree with crowdx there.

That said, I just hope you're doing this cause you want to, not cause of the difficulties a new all-paladin team faces. Once you get over that first little hump (and it can be dispiriting at times doing that), 5xPaladins can work wonders.

Still, a Druid would be nice because it would make a ranged healer - I'd consider leaving it at that, since any DPS ranged will add to your complexity even further.

The alternative though (just in case), would be to simply gear and spec the fifth paladin in holy. If it weren't so close to Cata, I'd definitely be doing that for my team.