i would personally add 1 or 3 yellow tangs, but seeing as you already have 2 other tangs, i would go with one, which still may get picked on by the other two that have since created their territories in your tank.
adding 2 tangs of the same body shape/color is asking for an issue. the tangs only have eachother to fight, head to head, leaving one of the two to take abuse. having 3 distributes the aggression and they divide out the abuse amongst eachother. hope that makes sense