U.S. stock index futures rose on Friday ahead of key data on the U.S. labor market, indicating the S&P 500 may reverse a four-day losing streak. The Labor Department releases the July employment report at 8:30 a.m (1230 GMT). U.S. employment probably only inched up in July as the economy struggled to regain momentum, strengthening expectations of additional monetary stimulus from the Federal Reserve. Non-farm payrolls likely rose 100,000 last month, according to a Reuters survey, after gaining 80,000 in June. The S&P 500 index has fallen more than 1.5 percent this week as investor hopes for further stimulus measures from central banks were dampened and a trading error at market maker Knight Capital Group Inc on Wednesday dealt another blow to confidence in market structure.