Rising for the fourth day in a row, the Sensex at the Bombay Stock Exchange gained 266 points, or 1.4 percent, to close at a nearly three-week high of 18,886, on sustained buying by investors, amid firm global markets. The Nifty of the National Stock Exchange advanced 79 points, or 1.4 percent, to 5,551.Stock markets in Japan, China, Hong Kong and Singapore gained between 0.1 percent and 2 percent.