Four Horsemen is by no doubt the hardest fight in the game in terms of the entire raid fully understanding their classes, mechanics in the the encounter.
One simple thing that made the encounter real hard doing the first kills and progression of Naxx was the void zones spawning from Lady Blaumeux
These where cast on a nearby player to Lady Blaumeux, and where either Visible or invisible, to avoid dying to these, you had to move as a tank, healer and ranged dps. Other way to dodge this could be to take one tick, and then know the exact position of the invisible void zone and call it out.
This is by some called an "RNG" to the encounter, however if the raid is placed correctly at the safe spot there wont be any void zones at either other places then the people attending Lady Blaumeux doing the encounter.
Sources for the information is here: http://www.forgottenaspects.com/forums/index.php?action=profile;u=29;area=showposts;start=135 http://lurkerlounge.com/forums/thread-3820-page-2.html
"12. The Four Horsemen. Having repaired invisible void zones, the patch has finally removed randomness from the encounter. Know what to do and when to do it and these guys aren't too tough, though they're still harder than nearly everything else. The fight's gotten a bit harder since mages don't have as much threat reduction, which makes the fight last longer, which increases the chance that someone blunders and dies."
Videos here:
https://www.youtube.com/watch?v=sljVITJREd8&t=129s Look around 9:45 there is written text of several people confirming Death to invisible void zones