Snapping four days of gains, the Sensex at the Bombay Stock Exchange closed unchanged, at 19,997 on Wednesday, after the recent huge rally.The Sensex had advanced over 1,750 points in the previous four trading sessions. But the Nifty at the National Stock Exchange rose 16 points, or 0.3 percent, to 5,913.Stock markets in Japan, China, Hong Kong, South Korea and Singapore ended mixed.