Posting its sixth straight day of losses, the Sensex at the Bombay Stock Exchange fell 57 points, or 0.3 percent, to close at a fresh four-month low of 18,736, amid weak global markets, today. The Sensex has now lost 835 points in the last six trading sessions. The Nifty at the National Stock Exchange dropped 7 points, or 0.1 percent, to 5,651.Stock markets in Japan, Hong Kong, Singapore and South Korea declined between 0.1 percent and 2.4 percent.