The Nikkei stock index sank over 4 percent early Monday, hit by selling of exporter and other issues after the yen strengthened following the victory of Shigeru Ishiba in the ruling Liberal Democratic Party leadership race late last week. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 1,512.85 points, or 3.80 percent, from Friday to 38,316.71. The broader Topix index was down 73.85 points, or 2.69 percent, at 2,667.09.