It's also worth noting that Thane Korth'azz (who runs to the near left corner) casts Meteor, which splits its damage amongst those it hits, so you want to hug up around him; Lady Blaumeux (far left) creates void zones, which you don't want to stand in; Sir Zeliek (far right) casts a chaining holy bolt, so spread out around him; and Baron Rivendare (near right) doesn't create any environmental hazards.
So how do you tank four mobs when you probably only have two or three tanks in the raid? Fortunately, the encounter is designed with flexibility. The two rear bosses -- Blaumeux and Zeliek -- cast magical damage on the closest player, meaning that anyone can tank them. By relegating a caster and healer(s) to each of the back bosses, you can keep them occupied while the raid kills the front two.
There are two approaches to dealing with Rivendare and Korth'azz. If you have the DPS, you can burn down Kor'thazz as fast as possible and not have to worry about switching. The more controlled approach is to switch after three marks, ensuring the raid is clumped up around Korth'azz in case of meteors -- the tanks need to run towards each other, taunt the boss they're not tanking, and then either run back to their original corner or swap corners (make sure you decide which of these you'll do beforehand!). Meanwhile at the back, at three marks, the casters and healers simply swap sides.
You need to continue switching at three marks until the bosses are dead, while avoiding the environmental hazards around each boss. Once tanks at the front are free, they can tank the mobs at the back in the normal manner (ensuring they are closer to the boss than melee and hunter pets).
If you're burning down Korth'azz, you don't need to split the raid up -- identify which tanks and healers will be taking the other three bosses and everyone else kills Korth'azz. If you're taking the more controlled approach, split half your DPS between the first two bosses. If you're going for the achievement, you need to try to evenly split up your DPS between all four and remember to communicate as they get low on health so that certain groups stop DPS if needed. On normal mode, it's a little harder to split the raid up evenly and you might need hybrids to help out with healing during either version of the fight.
Note that you can just pull by walking up to the bosses, and they'll run to their corners. However, if nobody is in range of one of them, a powerful AoE will hit the entire raid -- so position the players dealing with the rear bosses before you pull. (You can walk along the side walls and not aggro the bosses.)
When they die, the bosses' loot appears in a chest on the middle platform.
Differences between normal and heroic: No differences beyond health/damage of mobs.
Next week we'll finish up our Quarters guide with the Construct (Abomination) Quarter, so if you ever wondered what all the fuss was about Thaddius, watch this space...