Wild Slash’s ferocious ability applies to all damage that would be dealt that turn, including the damage Wild Slash deals.

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.

Ferocious abilities of instants and sorceries that don’t use the word “instead” provide an additional effect if you control a creature with power 4 or greater as they resolve.