Roadside bombs killed four NATO troops in Afghanistan, the international force said Saturday, adding to a violent summer as coalition forces step up patrols in the Taliban-dominated south. Separately, NATO reported that Afghan and foreign troops found nearly 2 tons (1.8 metric tons) of processed heroin, 1,800 pounds (800 kilograms) of opium and 200 pounds (90 kilograms) of ammonium nitrate Friday that could have been used to make 25 roadside bombs in the southern province of Helmand. The drugs had a street value in the United States of more than $38 million before taking into account the common practice of cutting them with other ingredients, which would exponentially increase the value, according to the U.S. Drug Enforcement Agency.