Heir of the Wilds

• WOTC Official Reminder Textofficial rules
  Deathtouch — Any amount of damage this deals to a creature is enough to destroy it.

• WWizards of the Coast2014-09-20
  Official Ruling
  Heir of the Wilds has a triggered ferocious ability with an intervening “if” clause. That ability will check if you control a creature with power 4 or greater whenever Heir of the Wilds attacks. If you don’t at that time, the ability won’t trigger at all. If it triggers, the ability will also check if you control a creature with power 4 or greater as it resolves. If you don’t at that time, the ability won’t do anything. Note that the creature with power 4 or greater you control as the ability triggers doesn’t necessarily have to be the same one you control as the ability resolves.

Rulings via Scryfall