1 prot and 4 ret is ridiculously survivable. The prot paladin generates boatloads of threat so usually it is the only one being damaged usually so there's 1 focus for healing. Divine storm's healing is enough passive healing to deal with splash damage from bosses on all the slaves and all healing needed for dealing with trash. Judgments of the Wise and glyphed Seal of Command provides enough mana regen for the ret paladins to just spam things without any regard for mana. You also can get all the aura improvements and blessing of might improvement from ret.

5 Prot would be *less* survivable - no passive healing from divine storm, no mana recovery until much, much later in the tree, too much switching of threat from one character to another so figuring out who needs to be healed is an issue. You'd run out of mana FAST so your DPS would be throttled back on boss fights or anything that takes any real time.

The only change I might make to the team would be swapping out one of the ret pallies for a shaman, but it isn't that big a deal. My team is steamrolling through everything in TBC so far - I think the only boss that gave me any trouble was the dragon in Ramparts, but now I have a strategy that works and it's been smooth sailing.