Rising for the fourth straight session, the Sensex at the Bombay Stock Exchange posted its biggest single-day gain this year, of 439 points, to 17,430, today, as investors cheered clarity on tax-avoidance rules, an upgrade of the Indian markets, and as global market gained ground. The Nifty shot up 130 points, or 2.5 percent, to 5,279. Stock markets in Japan, China, Hong Kong, South Korea, and Singapore rallied between 1 percent and 2 percent, on encouraging euro zone news.