The Sensex at the Bombay Stock Exchange closed with a marginal gain of 4 points, at 17,134, today. The Nifty at the National Stock Exchange rose barely 2 points, to 5,191. Both the National Stock Exchange and the Bombay Stock Exchange will conduct a special trading session of one-and-a-half hours, between 11:15 a.m and 12:45 p.m., tomorrow, in the F&O segment, due to the upgradation of the system. Stock markets in Japan, China and Hong Kong slipped about 0.4 percent, each.