Falling for the fourth straight session, the Sensex at the Bombay Stock Exchange dropped 210 points, or 1 percent, to close at 20,716, today, on concerns of a rate hike by the Reserve Bank, and amid speculation that the US Federal Reserve will soon start trimming its monetary stimulus. The Sensex has now lost over 610 points in four trading sessions. The Nifty at the National Stock Exchange fell 69 points, or 1.1 percent, to 6,168.Stock markets in Japan, China, Hong Kong, South Korea and Singapore ended mixed.