Gaining for the fourth straight session, the Sensex at the Bombay Stock Exchange surged 256 points, or 1.3 percent, to 20,529, on buying in IT stocks, after Infosys raised its annual sales forecast, and as global markets rallied, on friday. The Sensex had gained 378 points in the previous three trading sessions. The Nifty at the National Stock Exchange jumped 75 points, or 1.3 percent, to 6,096. Stock markets in Japan, China, Hong Kong, South Korea and Singapore gained between 0.3 percent and 1.7 percent.