The National Highway Traffic Safety Administration, which has come under fire for being too lax and slow in dealing with the defective air bags, also called on automakers to be ready to expand their recalls beyond the warm and humid regions believed to be the epicenter of the problem. The defective air bags, which can launch metal shards into car occupants, have been tied to at least four deaths and many serious injuries. “More can and should be done as soon as possible to prevent any further tragedies,” the agency’s deputy director, David Friedman wrote to auto makers.