(Updates with open) NEW YORK, Dec 22 - Stocks slipped at Monday's opening bell as a heightened U.S. terror alert during a short holiday week on Wall Street knocked investor sentiment down a peg after four straight weeks of gains. The blue-chip Dow Jones industrials <.DJI> eased 15 points, or 0.15 percent, to 10,263. The Standard & Poor's 500 <.SPX> inched down 2 points, or 0.20 percent, to 1,086. The technology-loaded Nasdaq Composite Index <.IXIC> shed 5 points, or 0.28 percent, to 1,945.//