Continuing to fall for the fourth straight session, the Sensex at the Bombay Stock Exchange lost a hefty 493 points, or 2.4 percent, to close below the 20,000 point level, at 19,692, today, on heavy selling by investors, on interest rate concerns. The Nifty slumped 144 points, or 2.4 percent, to end below the 6,000 level, at 5,905.