Typical 10-man Four Horsemen encounter requires three healers. Two in the front swapping position for debuff and one in the back healing both range tanks outside of debuff range.
However, when your raid comp is terrible and have only two healers, the encounter is still very much possible with skilled warlock pet tanking strategy.
You do so by sending in the voidwalker to melee the mob your are tanking at least once, then pulling it back out of melee range between the front group and the mob you are tanking. You and your pet need to both tag the boss (usually you start with a deathcoil) in order to be on the mob's aggor list. You then stand between your pet and the front group outside of the buff range and continue to health funnel your pet. Be sure to put on demon armor so the healers can occasionally top you off easily.
Once your pet's debuff stack to four, it will now become very difficult to keep your pet topped off. Simply run into range to take over, sacrifice the voidwalker so you have a protective shield around you. Summon another voidwalker, send it in to melee the mob once, then repeat the whole position again.
In this video you will see how your warlocks can effectively tank the encounter by utilizing all the Voidwalker abilities, thus allowing only two healers in front..