U.S. stocks stumbled early Friday, declining for the fourth day this week in volatile trading that has wiped out investors’ gains so far this year.The main forces behind the recent financial turmoil appear to be renewed concerns about China’s economy and uncertainty in Greece after the resignation of its prime minister. On Friday, China reported that its gauge of business fell unexpectedly, sending the Shanghai Composite Index into a steep 4.3 percent drop.Read full article