Even relatively "squishy" DPS in full DPS gear, DPS spec, DPS stance, etc. do not take sufficient damage to make it impossible (or even tremendously difficult) to heal through any 4 man content currently implemented, up through and including HM LI.
And, as a matter of fact, it's so much faster to clear 4 man content by foregoing a tank entirely that the only reasons to take one are if PUGing, if everyone is going on ungeared fresh 50 alts, or if a tank needs a clear and doesn't have the gear/experience to play their AC as a DPS.
This is true if you overgear the content. If you are running at an appropriate gear level, a tank is a necessity even if only to control where the incoming damage goes.