Trying to figure out a good benchmark for moving to WT4, I don't really want to switch it to WT4 then get 1 shot by the first mob I see. Anyone have anything that helps with this?

Literally as soon as you possibly can. The Capstone dungeon is a good metric. If you can easily get to the boss room, odds are you're ready for WT4. Just add some fire res, kill the boss. Gently farm helltide to start in WT4 and you'll power up quickly.

Bro it's d4, just use +hp and + res tempers, as simple as that.