A deepening crisis involving deadly air bags is shaking confidence in the ability of automakers including Toyota Motor Corp. and Honda Motor Co. to ensure the safety of millions of U.S. drivers. Toyota is advising U.S. owners to keep passengers out of front seats until defective Takata Corp. air bag parts are replaced, four months after following the same procedure in Japan. Takata faces investigations into whether exploding shrapnel from its flawed parts are to blame for at least four deaths involving vehicles made by Honda, including a Florida crash that was initially investigated as a homicide because of deep gashes to the victim’s neck.