I wouldn't mind the 4th tank being based on a similar model to WoW's Brewmaster Monk or Guardian Druid. Evasion based using either a huge health pool to mitigate bad streaks, with self-heals to supplement healers. Or a shunted damage pool that absorbs a percentage of damage taken, which then slowly bleeds the excess damage back to the player. Dancer could work as the latter, Beastmaster as the former.

Blood Death Knights use a combination of self-healing and damage bubble that took a while to fix, but lately has been very balanced in relevant challenging content. They use an attack that heals them for the amount of damage done, then creates an absorb shield on a percentage of that damage. Tends to break content that they over-gear. Would have said Dark Knight, but that ship has sailed so I have no idea what class could work

Just a few ways that tank mechanics could workout.