By Sakura Murakami and Elaine Lies TOKYO (Reuters) -Tens of thousands of Japanese households remained without power on Thursday morning after a magnitude 7.4 quake struck shortly before midnight, throwing a swathe of northeastern Japan into darkness, severing key transportation links and killing four. Companies including Toyota Motor Corp and chipmaker Renesas Electronics Corp raced to assess the impact, with any supply chain disruption likely to add pressure to strained global output of smartphones, electronics and automobiles. The temblor revived memories of Japan’s biggest quake, of magnitu…