LONDON (Reuters) - Stock futures pointed to a weaker open on Wall Street on Friday after four straight days of gains, with futures for the S&P 500, the Dow Jones and the Nasdaq 100 falling 0.1 to 0.2 percent. The Labor Department releases the March Producer Price Index at 7.30 a.m. ET. Economists forecast a 0.2 percent drop, compared with a 0.7 percent rise in February. U.S. March retail sales figures are due at 7.30 a.m. ET. Economists expect an unchanged reading, compared with a 1.1 percent increase in February. ...