Prowess triggers when you cast a noncreature spell, while the spell you just cast only checks whether Ferocious is true as it resolves. Since the trigger goes on top of the spell and resolves first, this generally works out well for you!
Tag: Force Away
Force Away on your only 4+ power creature – No looting for you.
What happens if you are saving your 4/4 from a removal spell using Force Away? Do you still get to draw and discard? Only if you also control another 4+ power creature, it turns out. Here's why.