I would think, a healer + tank + 3x melee DPS would be doable for pretty much everything.
You'd have the entire team in melee (aside from the healer), which means most of the puddles of acid will be in a smaller area, making them harder to avoid... and all four toons will likely eat point blank boss AoEs or the odd cleave.

I don't see 5x DPS, no healer spec among the group as being that viable initially with any class combination... you might be able to brute force through some things as Rets, but they would have a harder time then Tank + Healer + 3x Melee or 3x Range.