To be fair to the mouth-breathing yellow journalist who spewed forth that crap, I think it was saying that the four guns found at the scene, plus the stuff found in the house, was a near-apocalyptic arsenal. So that's, like, 6 whole guns, and at least 1600 rounds (probably doesn't include the ammo used/found at the scene), not to mention the terrifying BB gun, and those high-capacity assault swords.
Personally, with my much lower net worth than Mrs. Lanza's, I've managed to acquire 5 firearms. She was not very serious about that apocalypse stuff.