Falling for the fourth straight session, the Sensex at the Bombay Stock Exchange declined 12 points, or 0.1 percent, to close at a fresh four-month low of 18,438, amid weak Asian markets on Monday.The Sensex has now fallen more than 600 points in four trading sessions. The Nifty at the National Stock Exchange lost 10 points, or 0.2 percent, to 5,543.Stock markets in China, Hong Kong, South Korea and Singapore had dropped between 0.1 percent and 0.6 percent.