Walt Disney's DIS.N shares dropped 1.6% ahead of its second-quarter results due after the closing bell. By Medha Singh May 13 (Reuters) - U.S. stock index futures fell for the fourth session on Thursday as investors awaited producer prices data, another inflation gauge, to see if a rise in prices would be strong enough to prompt a sooner-than-expected increase in interest rates. The CBOE volatility index .VIX, a measure of investor anxiety, has surged to its highest levels in more than two months to 27.63 points.