This isn't really a terrible card despite its initial impact. Yes, it's a 2/2 for 4 with no evasion. On that basis alone, it's pretty bad, but there are other factors here. Imagine it as a Shock stapled to a Forest Bear. It`s like that other 2/2 for 4 that does damage when it dies: Shock Troops. The main difference of course being that you can blow up Shock Troops whenever you want and it can target creatures. This card has to die by some other means and can't hit creatures, but it gets a bonus to attack and its damage is variable. It basically asks your opponent if they want to keep taking 4 damage, discard a card and lose 2 life, or sacrifice a creature (probably) and lose 4 life.
Pros: Opponent doesn't want to get rid of it, bonus when attacking
Cons: A 2/2 for 4, no evasion, can't target creatures