Tanks rely on vengeance to deal good dmg, and you build up vengeance by taking dmg (before mitigation). Vengeance lasts 20s ... so you would need a 5s taunt rotation to benefit from it. The problem however is that as a tank you don't take every 5seconds a big hit.

The only reasons that I can see why you would want to run 4 tanks is because you either ...
... want easymode and not care about the time it takes to kill. You can avoid a lot of fightmechanics/brute force stuff with running 4 tanks, since they have the best survivability in the game
... for lols.

There can definitely be a benefit to run with 4 tanks, since you can game the vengeance. But there is no way that you can keep rotating vengeance with 4 tanks, besides maybe a handful of gimmick bosses.