Making -4 an automatic failure might make a philosophical point about certainty and your gaming philosophy, but it won't affect the game a whole lot. So I guess it might be a good idea if you're aiming for symbolism.

But even at Chest Deep, a Stunt can easily make someone Epic at some variety of Declaration. So the issue can arise in normal games.

I personally feel a game that is designed around telling a story should have that very principle taken to heart. Turning this story-driven game into a game of gimping a balanced rolling system is not the answer: As Sanctaphrax pointed out, it won't affect the game a whole lot, but what it will do is change balance that could've been avoided by making them describe their declaration before the GM gives them a difficulty. Because by making four minuses an automatic fail without making four pluses an automatic success, you're punishing your players for bad rolls without rewarding them for good ones.

Judgement should be used, as a GM, to say no for the sake of preventing powergaming, as aforementioned by dragoonbuster by simply increasing the difficulty based on likelihood of that declaration happening, or a whim for the sake of making the game challenging. This is, after all, a game we all play to enjoy; not to struggle and be frustrated with. There will always be roleplaying dilemmas that make our character's very core tried and tested; that shouldn't be left up to dice rolls for a story-driven RPG, in my honest opinion.