TOKYO, Nov 17 - Japan's Nikkei stock average slid almost four percent on Monday, undermined by fears about the naming of Japan as a possible target in an al Qaeda statement and after disappointing corporate results helped dent U.S. stocks. The Nikkei fell below 10,000, a level widely expected to provide a solid floor, and triggered a wave of stop-loss selling especially in banks and other issues that recently made large gains on the back of recovery in the Japanese economy. Individual investors continued to dump shares that had helped power the Tokyo market's six-month rally before the latest downturn, sending Internet services firm Softbank Corp <9984.T> down by its daily 500 yen limit to 3,500 yen. The Nikkei 225 stock average <.N225> finished down 3.74 percent at 9,786.83, its biggest one-day fall in a month. It was the first close below 10,000 since August 15. The broader TOPIX index <.TOPX> dropped 3.46 percent to 971.89.//