Is there a catch? Of course there is: the $4 flight routes won't be posted online until July 3rd, one day before the flight itself, meaning you'll have to be pretty flexible with regard to where you're actually going -- recent routes have included Teterboro to Palm Beach and Los Angeles to Las Vegas. Also, these are one-way flights, so you'll be on your own figuring out an itinerary for your return trip.
Still, at $4 you can afford to pay the trumped-up fees for your last-minute Spirit flight back home. To get a leg up on the competition, register for SuiteDeals and set up a Wishlist that'll shoot you an email or text message if there's an available flight posted from one of your chosen airports.