Daihatsu, a unit of Japanese automaker Toyota Motor Corp., has shut down production lines at all its four factories in Japan while transport ministry officials investigate improper tests for safety certifications. The shutdown as of Tuesday comes a week after Daihatsu Motor Co. announced it was suspending all vehicle shipments in and outside Japan after finding improper testing involving 64 models. The safety test irregularities earlier this year triggered an independent panel investigation, which found widespread and systematic problems at Daihatsu.