Multiple Questions:

1. With Ferrothorn Sassy Evs 255/def and sp. def (I thought I was keeping track and somehow I think that happened) I am considering putting Toxic with it. Current moves

Poison Jab

Iron Defense

Power Whip

Curse

Should ID or Curse be replace with Toxic or keep as is?

Two. What is the mathematical formula for Ev training with pokerus and +4 ev item?

Ive read (Ev of defeated pkmn + '+4 item') X 2. Is this correct?