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 key 20,000 point level, at 19,692, today, on heavy selling by investors, on concerns about an impending interest rate hike. The Nifty at the National Stock Exchange slumped 144 points, or 2.4 percent, to end below the 6,000 level, at 5,905.Stock markets in Japan, China, Hong Kong, South Korea and Singapore once again ended mixed.