Falling for the fourth straight session, the Sensex at the Bombay Stock Exchange lost 72 points, or 0.5 percent, to a 20-month low of 15,792, on sustained selling by investors, amid weak Asian markets, today. The Sensex had fallen more than 830 points in the previous three trading sessions.The Nifty at the National Stock Exchange declined 21 points, or 0.4 percent, to 4,751. Stock markets in Japan, Singapore and South Korea dropped between 0.1 percent and 2.3 percent.