EXCLUSIVE: At least four classified Hillary Clinton emails had their markings changed to a category that shields the content from Congress and the public, Fox News has learned, in what State Department whistleblowers believed to be an effort to hide the true extent of classified information on the former secretary of state’s server. The changes, which came to light after the first tranche of 296 Benghazi emails was released in May, was confirmed by two sources -- one congressional, the other intelligence. The four emails originally were marked classified after a review by career officials at the State Department. But...