Problem in back? Tag the mobs.
I had this same problem myself. It really seems like a bug, but either way the solution is this:
Originally Posted by DarthRegis
The two "tanks" start off close to the corners in the back. Somebody makes the pull and the 4H run to their corners. As soon as they get to the back, both of the people in the back must tag them somehow. Throw a DoT, a ranged weapon, whatever. As a holy priest I just throw a Shadow Word: Pain.
At that point, you should have aggro and they should begin wailing on the two back "tanks" just the way you expect. Then you can move backward toward the center of the back strip so that the healer can target both people at the same time. You can get pretty far; damn near to the center of the back, but as long as the healer back there can target both players it's good enough.
You do NOT need to tag the mobs again when you switch. After the initial pull + tag, everything works much as you expect. One quick tip is to make sure you're relatively well healed-up before you switch; you can easily take 2-4 hits in the back before you switch, get to the other side, plant and get your next heal off (if you don't have many instant heals).
After that just remember that in the back, your only job is to live long enough for the front to come down and help you. Don't worry about DPS or anything; anything you do back there is just a bonus. If you have a pet of any sort, you can use it. As a holy priest I usually start out in the back right corner, and the second time I get to the back left I throw my shadowfiend. In addition to getting some much-needed mana at that point, he also tanks Lady while he's out (and since she's shadow damage, he seems to last longer on that side--but maybe that's my imagination). It's a nice breather to only focus on one person back there for 10 seconds.
4 Horsemen 2 healer strat
My guild never always has the correct grp makeup for this fight... we have to have our Disc priest in the back usually with a hunter or ret pally. We have our Resto Druid with the OT and the rest of the DPS on Thane. As the MT and a Prot Pally, I have come across a great way to be able to solo Baron so the rest of my grp can take Thane down in one shot, and we dont have Bloodlust, or Heroism for those Ally players. First off it is kinda based off of the Paladin's Spl Pwr and I don't advise anyother tank try this. I have over 41k hp fully raid buffed, so my spl pwr is usually over about 800. What the Pally tank should do is Seal Light and judge light. save your mana for heals after that... It's really easy in the begining but you will have to use a few holy lights on yourself. As a Prot Pally you should have that talent that grants you 30% more spl pwr based on your stam, so you should have 30% more healed on your crits. Its nice to get lucky and crit heal yourself for about 14k but its not required for this to work. Now after Thane is down and the main grp is moving over to Baron I take my healer and head to the back as my main grp with our Resto Druid can handle atleast 5 "Marks" before trouble happens. I look at my raid bars and see who is in the most trouble and head that way. My guild developed this strat only because we had a new raider in out grp that didnt understand to stay at a ranged distance in the back so we did wipe about 5 times before we did this strat... And now that he has it down we never use anything else and we always one shot 4HM.