Right now some hybrid classes benefit from wearing a pair of 2 piece set bonuses in PvP instead of their 4 piece bonus. We don't much like that behavior though, and we're going to eventually make the two bonuses exclusive so you can only ever have one 2 piece bonus at a time. We're not going to make that change in 4.0.6 as we realize some people have spent a lot of points to buy into the double-2 piece bonuses. However, it is likely we'll make the change at the start of the next PvP season.
I know this will bug people. I don't care much personally. I generally wear the 4pc anyway. I'd have gone for the double 2-piece after the incoming Int nerf, except this is too good to pass up when combined with Body and Soul:
The PvP priest 4-piece set bonus has been redesigned. When Power Word: Shield is cast on the priest, movement speed reduction effects are temporally suppressed for 4 seconds.