Falling for the fourth straight session, the Sensex at the Bombay Stock Exchange lost 236 points, or 1.4 percent, to close below the 17,000 mark after three weeks, at 16,883, on continued selling pressure, amid weak Asian and European markets, today.The Sensex had already lost over 450 points in the past three trading sessions. The Nifty at the National Stock Exchange fell 80 points, or 1.6 percent, to 5,069.Stock markets in Japan, Hong Kong, South Korea and Singapore fell between 0.7 percent and 0.9 percent, on fresh Euro-zone concerns.US stocks ended modestly lower, on Monday. So the Dow Jones Industrial Average fell 75 points or 0.6%, to 12,077.The NASDAQ Composite index lost 22 points, or 0.8%, to 2,656. And the S&P 500 lost 12 points, or 1%, to 1,251.Twenty-seven of the 30 Dow components lower, led by JP Morgan Chase and Bank of America, each of which shed almost 2%.Leading advancers on the blue-chip index, Boeing climbed 2.6% after the airplane manufacturer snagged a 26 billion dollar order from Oman Air.On the quarterly corporate results front, in-line earnings from J.C. Penney were overshadowed by a disappointing forecast.