Did you check all the little posts and tabs next to the pulleys to see if they bent away from the pulley enough for the belt to slip off? Usually these are flat tabs on the deck, and round thin posts on the engine pulley. I don't have a Husqvarna in the drive right now to look at.
On second thought, 4 inches slack in an awful lot. I would double check the routing of the belt. Sometimes there is a sticker on the deck.