Rising for the fourth straight session, the Sensex at the Bombay Stock Exchange surged 227 points, or 1.2 percent, to close at a six-week high of 19,407, amid hopes of a rate cut and a sovereign rating upgrade, and rising Asian markets, today. The Sensex has now gained 675 points in the last four trading sessions. The Nifty at the National Stock Exchange jumped 79 points, or 1.4 percent, to 5,916.Stock markets in Japan, Hong Kong, South Korea and Singapore gained between 0.5 percent and 1 percent.