Extending its gains for the fourth straight session, the Sensex at the Bombay Stock Exchange surged 270 points, or 1.4 percent, to close at a one-month high of 19,683, amid positive European markets, today. The Sensex had gained over 535 points in the previous three trading sessions. The Nifty at the National Stock Exchange climbed 82 points, or 1.4 percent, to 5,946.Stock markets in Japan, China, Hong Kong, South Korea and Singapore again ended mixed.