A potential safety crisis over defective air bags widened Monday as the U.S. government issued an urgent plea to more than 4.7 million people to get their cars fixed.The inflator mechanisms in the air bags can rupture, causing metal fragments to fly out when the bags are deployed in crashes.Exploding airbags continue to injure drivers, even after major recall Safety advocates say at least four people have died from the problem and there have been multiple injuries. They also say more than 20 million vehicles in the U.S. are equipped with the faulty air bags.