That sounds like you had an under geared tank in T4 to be honest. Tanks shouldn't be taking damage that fast there.

As far as T5 you're getting ready for your first real challenge as a healer. Everything has been cakewalk up to this fight. Luckily it's not too hard now with 20% echo but the mechanics still may be difficult. I'd highly recommend text guides to accompany videos to fully understand the fight.