* Shutdown enters fourth day, investors watch debt ceiling
* Dow, S&P 500 on track for second straight week of losses
* Adobe Systems down; hackers stole source code, client data
* Futures up: Dow 6 pts, S&P 500 2.7 pts, Nasdaq 4.75 pts
By Ryan Vlastelica
NEW YORK, Oct 4 (Reuters) - U.S. stock index futures pointed to a slightly higher open on Friday, though the uncertain outcome of a partial U.S. government shutdown continued to worry investors.
The S&P 500 has fallen for nine of the past 11 sessions, with the losses driven by a budget stalemate in Congress that has closed the government since midnight Monday.
The shutdown, which is now in its fourth day, appeared likely to drag on for another week or more, with little sign of progress towards a solution. Investors are also watching the situation for how the upcoming debate over the debt ceiling might play out.
While a short-term closure was expected to have a modest economic impact, the effect is expected to become more pronounced the longer it lasts.
Dennis Lockhart, president of the Federal Reserve Bank of Atlanta, said the shutdown would hurt growth in the last quarter of this year, while the Bank of Japan warned that a prolonged budget standoff would have a severe global impact.
"We're oversold, but is this a bounce with some muscle, or just a feeble move? This isn't a sign of health; a resolution would give us a strong bounce," said Adam Sarhan, chief executive of Sarhan Capital in New York. "Every day the shutdown continues, it reduces the chances of a strong fourth-quarter earnings season."
The debt ceiling issue is considered more severe, as it could result in a default on U.S. debt if no resolution is reached to increase it. On Thursday, the New York Times reported that House Speaker John Boehner told colleagues he would not let the United States default on its debt.
S&P 500 futures rose 2.7 points and were slightly above fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures added 6 points and Nasdaq 100 futures rose 4.75 points.
While shutdown concerns have pressured equities over the past two weeks, the S&P 500 index has frequently found support at its 50-day moving average of 1,679.84, though it closed slightly below that level on Thursday.
For the week, the Dow is down 1.7 percent and the S&P has slipped 0.8 percent for a second week of losses for both indexes, while the Nasdaq has shed 0.2 percent.
The CBOE Volatility Index, a measure of investor anxiety, jumped as high as 18.71 on Thursday, its highest since late June. The VIX had jumped 160 percent to 42.96 in the third quarter of 2011 as the S&P 500 index fell 14 percent, the biggest retreat since 2008.
Government economic data has been delayed because of the shutdown, and the September payroll report was not released as scheduled. About 180,000 jobs were expected to have been added in September, up from 169,000 added in the previous month.
"Not getting key economic data really disrupts a normal investment process. When you remove a key piece of data, it becomes very difficult to navigate an already difficult environment," Sarhan said.
Adobe Systems Inc shares fell 2.1 percent to $49.79 in premarket trading. On Thursday, the company said hackers had stolen source code to some of its most popular software as well as the confidential information of millions of its customers.
Twitter Inc gave potential investors their first glance at its financials on Thursday when it publicly filed documents for an initial public offering. The information showed that revenue at the social networking company almost tripled in 2012, though it posted a loss in the first half of 2013.
Potbelly Corp said late Thursday its initial public offering of 7.5 million shares had priced at $14 each.
Union Pacific Corp gave a third-quarter earnings outlook late Thursday that was below expectations, as flooding in Colorado weighed on coal shipments.