I know that Tal Rasha 2pc gets procced by any elemental damage on enemies, whether they are from you, your pets (Familiar / Hydra), or Etched Sigil. (I'm not sure about your follower.)
And I know that Tal Rasha 6pc gets procced from elemental attack skills that are cast by you (regardless of whether they actually hit anything), so they are not procced by attacks from your pets or Etched Sigil.
But how about Tal Rasha 4pc? How is it procced? Can they be procced by your pets or Etched Sigil?