Wild Slash's ferocious ability applies to all damage that would be dealt that turn, including the damage Wild Slash deals.
24-11-2014
wotc
If you control a creature with power 4 or greater as Wild Slash resolves, the ferocious ability will apply. Damage dealt that turn can't be prevented, even if you no longer control a creature with power 4 or greater as that damage would be dealt.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.