I've been quad boxing Deathknight and so far it's going good in PvE, I have my doubts in PvP encounters though. Here my general strategy: I have my main DK and a swarm of others all /following the main. One of the DK's from the swarm pulls aggro with Death Grip, this gets the mob into melee range of the entire group of 4 DK's (Or as I call it, "The blender")

Death grip's cooldown is a little annoying, so I modified a Frost Nova macro I read about somewhere (Might have been wowwiki)
DK1's macro: /castsequence reset=75 Death Grip,,
DK2's macro: /castsequence reset=75 ,Death Grip,
DK3's macro: /castsequence reset=75 ,,Death Grip

What this will do is have it so that each time (within a 75 second time frame) you press the macro, only 1 DK will cast DeathGrip (Instead of all at once which would be a waste)

This strategy has been working very well for me, good luck