The Nikkei stock index on Wednesday plummeted more than 4 percent, or 1,600 points, its third-largest point drop of the year, as weaker-than-expected U.S. manufacturing data raised concern about a U.S. economic slowdown. The 225-issue Nikkei Stock Average ended down 1,638.70 points, or 4.24 percent, from Tuesday at 37,047.61, its lowest level since Aug. 15. The broader Topix index finished 99.78 points, or 3.65 percent, lower at 2,633.49. Every sector on the top-tier Prime Market lost ground, led by mining, nonferrous metal, and securities house issues.