Switch one pala to ranged dps

Hello .
I multibox 5 toons on a private WOTLK server.
Only PVE.
I started with a dk tank / 4 ret paladins team.
Then i switched one ret paladin to holy to have a dedicated healer.
Now as i clear more and more instances i have the feeling that a lot of boss mechanics are hard to avoid with 4 melee toons...( Boss AOE, AOE debuff on a toon and so on ...)
So i would like to switch a ret paladin to a ranged dps. Maybe it would make my heal easier and my survivability better.
So which ranged class would benefit to my team ? Which ranged class is easier enough to multibox and profit from my DK and paladin buffs ?

No need to switch any of your palys to holy. 4 ret palys do enough single target heal with instant flash heal through art of war procs and AoE heal with divine storm. If you still lack heal on some boss encounters you should switch to caster 1hand and shield on some of your palys.