Falling for the fourth straight session, the Sensex at the Bombay Stock Exchange declined 42 points, or 0.2 percent, to close at 20,684, on profit-booking by investors, amid falling Asian markets, Friday. The Sensex had lost 169 points in the previous three trading sessions. The Nifty at the National Stock Exchange fell 19 points, or 0.3 percent, to 6,145.Stock markets in Japan, China, Hong Kong, South Korea and Singapore lost between 0.4 percent and 2.8 percent.