Unlike the buoyant U.S. stock market, the dollar is sinking fast.  The value of the greenback just hit a four-year low, according to the ICE U.S. Dollar Index, the Intercontinental Exchange's measure of the currency against a basket of six other major currencies. While much of the decline has occurred over the past year, the dollar has slid more than 3% since mid-January, according to the index.  The dollar's slide impacts everything from financial markets to people's overseas vacation plans.