The Nikkei stock index plunged nearly 5 percent Monday, as exporters were sold on a stronger yen since Shigeru Ishiba won the ruling Liberal Democratic Party’s leadership race late last week. The 225-issue Nikkei Stock Average ended down 1,910.01 points, or 4.80 percent, from Friday at 37,919.55. The broader Topix index finished 95.00 points, or 3.47 percent, lower at 2,645.94.