Why would you ever cast this if your opponent hasn't been dealt 4 damage?

No idea. Needed an "otherwise, do this" clause. Still playing around with the idea of red conditional counterspells.

Have to admit, that is a very weird 'otherwise, do this' clause. In theory, it would be relevent if you cast this, then, say Chared your opponent, only to have that player cast a damage prevention spell. But if you just reversed the order - Charred your opponent, waited for it to resolve, then cast Explosive Failure, you avoid that scenario. Seems like a trap for bad player, or a way to deal 4 damage to yourself, and that's it.