The question was 4-man pvp.
It did not specify BG's, Arena with a Healer, Rated BG's, World PvP, etc.

4x Rets was the first suggestion.
But 4x Frost DK's was the second.

Between the two, the main advantage seems to be:

The DK's can do a lot of damage at range.
Howling Blast, is a 10 yard AoE with the initial target up to 30 yards away.
So you can in theory hit someone 40 yards away, if they're in the splash burst.
It applies a disease and snares.
And a Frost DK can open with four Howling Blasts, without taking Runic Empowerment into consideration.

The Paladins have higher burst once they're in melee range.
And their survival is also higher.
With what we know of them for MoP, they'll perform similar to their pre-Holy Power era.
With no cooldown on Judgement, and two Judgements giving an instant Flash of Light (x4 or x5)...
That's basically a full heal every third global, not even looking at Word of Glory or other talents.
The changes to Exorcism, make it an instant cast so they don't need to mess around with Art of War procs and resetting the swing timer, etc.
5x Rets might have the survivability of 4x Frost DK's + 1x Healer (all five of each played by the same player).

Both classes have a few new toys.
And will be fun compositions.
Prior to the 4x DK & 1x Healer composition, 4x Ret's & 1x Frost DK was top dog (and was only dethroned, because of Holy Power messing up the rotation).
Kromtor hit 2600 is 5's Arena with the 4x Ret/1x DK.