IF you have a tank that can solo-tank the boss, there is really no point whatsoever in bringing a second tank. The DPS that a second tank brings over a 4th healer is almost neglectable considering the dps-checks you want to achieve. If you can cut a tank, do it. I hope your roster has a druid or monk.
So why bring 4 healers instead of 3?
This is not a question of necessity, but rather of progression speed. You do not really *need* a 4th healer, but it will prevent a very significant amount of deaths from icy/fel ejections and therefore wipes. We realized after killing it that 1xx wipes to the boss made our healing so efficient that we could have easily cut the 4th healer after progression ended, but when progressing the boss started we initially had issues with people dying to any of the dots.
So what adding a 4th healer does is taking a lot of risk out of the progression equation, obviously. It also deletes the time your healers need to learn the fight before you can actually progress a phase dps-wise. You just overheal it.
So when considering how to get 15 DPS into the little room, I would recommend 4 healers and 1 tank. That requies a good tank though. When you can 3 heal it, the only reason to bring a second tank is because your tank isnt capable of solo-tanking the boss.