First time playing in season 4. Finally got all my renown done, hit 100, have decent gear at 8/12 masterwork.

Where do I go from here to chase the best drops? Curious to know between pits / nm dungeons / helltides / bosses what is the most efficient way to do some magic finding.

Helltides for quantity with a slim chance at 2 GA legendaries.

Tormented bosses or high level pits for 2 GA+ and GA uniques

Is high level considered over 100 only?

hell tide blood maidens if you're purely looking for loot, open chest in between summons.

The highest you can comfortably run. I would aim for 75+ though. You have the added benefit of the occasional stygian stone as well from Pits which can support running tormented bosses or consistently sold for 30m-35m