Rising for the fourth straight session, the Sensex at the Bombay Stock Exchange posted a modest gain of 49 points, or 0.3 per cent, to end at a two-and-a-half-year high of 17,986, today, amid firm European markets. The Nifty at the National Stock Exchange rose 18 points, or 0.3 percent, at 5,401. Stock markets in Japan, China, Hong Kong, South Korea, and Singapore ended mixed.