Of any 4 of one class for matchmaking to build, it went with Dragonknight, the most default tanky class we have to choose from and therefore the most overpowered in the current state of pvp. Here's the problems that really get highlighted when you got 4 of these things as opponents:
1) 4 leaps to deal with. Unless you're a freshly shielded, 100% hp bruiser or tank yourself, these homing missiles will knock you up (multiple times in fact) and altogether will guarantee your death--and your nearby teammates. There's no counter to this like if it were 4 Nightblades trying to Incap/SH you or 4 Templar whirling their spears around or 4 Necromancers dropping Colossi. No other single ultimate can make you feel so powerful even if you have zero additional offensive stats.
2) Petrify. If 4 DKs all press their favorite Petrify button on you at the same time (which they will) you are gone. Confusingly, DKs get all the skills that cannot be dodged, blocked, or reflected. Why? Who thought it was good to focus all of these skills onto one class? And why does Petrify do so much hard CC in addition to damage? Nothing compares to the strength of this skill.
3) Breath. Breath. Breath. The only other comparable skill to an AoE Major Breach is Warden's Deep Fissue, which again, does not compare. Deep Fissure needs to be aimed from its thin line 3 seconds after you press the button. Noxious Breath is a conal instant cast with a DoT and resource regen... Um, what? Not like Caltrops or Unnerving Boneyard are good substitutes either because you can just dodge roll out and you're back to full armor. And don't even get me started on how just 1 Engulfing Flames morph adds 40% additional damage to you in this scenario.
4) Lava Whip--a guaranteed buff. You'll rarely see a DK not proc the buffed version of either morph, but you especially won't when there's 4 of these things spamming Petrify on you. Why is there even a buffed version if that's all that a DK will ever utilize because of the ease of accessing it? And now to critique the morphs: Molten Whip, a mini Frenzied Momentum for free. Flame Lash, a giant heal x3 before you manage to break free (with server lag of course) and get out of the immobilized state. Wonderful.
5) Dragon Blood. It doesn't matter which morph is chosen; the fact that this skill heals a flat percentage of missing health is preposterous. Just as you think you're bringing down this over-compensated class to low health, they press 1 button and your hard work is undone. And they'll be waiting to use it again because of course they have plenty of resources leftover from all the poisons and flames they've spit on you.
The DKs that I always see 1vEveryone use each of these skills, seemingly with any armor (I'm not complaining about Plaguebreak--the class itself is the problem), always with dual wields using Whirling Blade, an AoE execute that you'd be foolish to think DK is bereft of (and this execute hits through dodge roll--great, another undodgeable addition). Occasionally you'll see one rocking a morph of Ash Cloud with it's oh so cute SEVENTY percentage snare (highest in the game, right?). And if you think, "Well, at least they don't have travel abilities". I give you Fiery Grip (and it's morphs) or the commonly seen Undo from Psijic.
Dragonknights have been built with no regard in mind for their utility and strength. I cannot find a weakness in their setup besides purely overwhelming them. I do think cost-increase poisons would help deter them, but no 1v1 build is going to rock that--only a support might. Sure would be great to have that be accessible on ANYTHING ELSE. And I think the best counter is break free armor sets like Unchained Aggressor, Zoal, and Indomitable Fury, but it's still not going to be a hard counter; that's just reactionary to what you can expect to be pummeled with.
I do believe any class can warrant a "DIS OP DAWG" thread. However, it appears the skill curve to achieve greatness with a DK is simply the same wombo combo of petrify>breath>whip>leap--which is easily and often done with no drawbacks.