Extending its gains for the fourth straight session, the Sensex at the Bombay Stock Exchange rose 111 points, or 0.6 percent, to 17,504, on sustained buying by investors, amid firm European markets, today. The Sensex had gained nearly 300 points in the previous three trading sessions. The Nifty at the National Stock Exchange advanced 32 points, or 0.6 percent, to 5,332.Stock markets in Japan, China, Hong Kong, South Korea and Singapore ended