The biggest reason imho why 4rets work is the 4x divine storm + 4x Art of War instant Flash of Lights.
In heroics you WILL need healing, but you can do withouth a dedicated healer by substituting passive heals be it 4x totem of healing or 4x divine storm.

When you only take 1 or 2 rets there might be times on certain heavy hitting bosses that your tank goes down in a big spike.

Also this is the reason why many run DKtank + 4rets to add in the +20% haste buff (frost specced DK) withouth having to get a DK or Shaman DPS.

At first i ran protpally/4ret which worked quite nice in heroics, prot pally's are so sturdy i didnt even use my Art of War procs on healing but instead on Excorcism for dps.
But in PVP you notice you have no snare, but i must say it was fun driving my protpally in pvp cause he could soak up a LOT of damage

Than i switched it up to a DK tank for PVP (deathgrip, chains of ice, chillblains slow) and this made it possible to do better in PVP.
In heroics im still doing pretty damn good but i had to start using my Art of War procs on Flash of Lights on the DK cause DK tanks tend to get more spikyer damage than a prot pally.