I'd run 4x ret + dk because I enjoyed that comp. It also gives you a few more options as to making sure your rets stay on target and a means to do some damage. I think though that 4x ret + healer will be really good in MoP, but then again i'll be running double ret + dk in 3's.

If you don't have a means to get rid of HoP or other similar effects its going to be hard to actually get a kill as a ret team even in MoP. Which is why I'll be using a DK due to their new purge mechanic as well as their grip, chains for root, silence, and ability to do 50k damage if someone is below 35% though I might try a warrior.

If I do play MoP that is.