By Gareth Jones and Aleksandar Vasovic KIEV/MARIUPOL Ukraine (Reuters) - Four Ukrainian servicemen have been killed since the start of a ceasefire between government forces and pro-Russian separatists last Friday in eastern Ukraine, a defense ministry official was quoted on Tuesday as saying. The ceasefire is part of a peace plan intended to end a five-month-old conflict the United Nations says has killed more than 3,000 people. It has also caused the sharpest confrontation between Russia and the West since the Cold War. ...