Extending its gains for the fourth straight session, the Sensex at the Bombay Stock Exchange rose 21 points, or 0.1 percent, to 17,257, today. The Sensex had gained almost 600 points in the previous three trading sessions. The Nifty at the National Stock Exchange added 12 points, or 0.2 percent, to 5,241.Stock markets in Japan, China, Hong Kong, Singapore and South Korea ended mixed.