Falling for the fourth straight session, the Sensex at the Bombay Stock Exchange dropped 91 points, or 0.5 percent, to close at 19,660, on profit-booking by investors, amid weak regional markets, today. The Sensex had lost over 250 points in the previous three trading sessions. The Nifty at the National Stock Exchange fell 30 points, or 0.5 percent, to 5,957.Stock markets in Japan, Hong Kong, South Korea and Singapore declined between 0.8 percent and 2.3 percent.