NUEVO LAREDO, Mexico, March 7 (Reuters) - Suspected drug hit men on Tuesday killed four policemen in a shootout in this crime-ridden Mexican city on the U.S. border, police and witnesses said. One of the dead was the second-in-charge of a state police force sent to Nuevo Laredo last year by President Vicente Fox's government to help end a drug feud, the sources said. The killings took place in broad daylight on a busy highway leading to the airport in Nuevo Laredo, across the Rio Grande from Laredo, Texas. The policemen and their attackers fought for 15 minutes in a...