* IBM slumps after quarterly results
* Apple earnings due after the close
* Futures off: Dow 90 pts, S&P 5.5 pts, Nasdaq 4.5 pts (Updates prices, adds premarket actives)
By Chuck Mikolajczak
NEW YORK, Oct 20 (Reuters) - U.S. stock index futures were lower on Monday, on the heels of a fourth week of losses for the benchmark S&P 500, as quarterly results from IBM disappointed.
IBM shares slumped 7 percent to $169.24 as the most actively traded stock on the New York Stock Exchange in premarket after the company's third-quarter earnings fell well short of Wall Street expectations. IBM had earlier said it would pay Globalfoundries $1.5 billion in cash over the next three years to take its loss-making semiconductor unit.
The S&P 500 rallied on Friday to pare its weekly decline, but still notched its longest stretch of weekly losses since August 2011. The S&P index is now down 6.2 percent from its record high, hurt by worries about the health of the global economy, the spread of the Ebola virus and uncertainty about the next steps for the Federal Reserve.
Earnings season will ramp up significantly this week, with nearly 130 S&P 500 companies scheduled to report, include Apple Inc after the close on Monday.
According to Thomson Reuters data through Friday, of 81 companies in the S&P 500 that have reported quarterly earnings, 64 percent beat analyst expectations, slightly above the 63 percent average since 1994 but below the 67 percent rate for the past four quarters.
Third-quarter earnings are expected to grow 6.9 percent from a year ago, on revenue growth of 3.8 percent.
Halliburton shares advanced 3.8 percent to $54.60 before the opening bell after the world's No.2 oilfield services provider, reported a 70 percent rise in net income and boosted its quarterly dividend.
NewLink Genetics Corp surged 29.3 percent to $37.96 before the opening bell. The company said it entered a licensing agreement with Roche Holding AG unit Genentech to develop NewLink's cancer immunotherapy, making the Ebola vaccine developer eligible to receive over $1 billion in milestone payments. Volume of nearly 50,000 shares represented over 6 percent of its 50-dAY average.
* S&P 500 e-minis were down 5.5 points, or 0.29 percent, with 273,781 contracts changing hands.
* Nasdaq 100 e-minis were down 4.5 points, or 0.12 percent, in volume of 35,961 contracts.
* Dow e-minis were down 90 points, or 0.55 percent, with 45,598 contracts changing hands. (Editing by Bernadette Baum)