BOSTON — A commuter ferry ran aground in fog in Boston Harbor on Friday, injuring four people on board. The captain of the MV Lightning was forced to take evasive action to avoid a collision after being “cut off” by a sport fishing boat, said Joe Pesaturo, a spokesman for the Massachusetts Bay Transportation Authority. The ferry, which was traveling to Boston from Hull with 84 passengers and crew members, struck some rocks and ran aground in about five feet of water near Long Island, one of several islands in the harbor, at about 7:45 a.m.