Falling for the fourth straight session, the Sensex at the Bombay Stock Exchange dropped 157 points, or 0.8 percent, to close at 20,666, on rupee weakness, and amid declining global markets, today. The Sensex has now lost 574 points in four trading sessions. The Nifty at the National Stock Exchange fell 47 points, or 0.8 percent, to 6,141.