A 4-year-old girl fell 20 feet from an apartment window in Foxborough on Sunday and was flown to Massachusetts General Hospital by helicopter, fire officials said. The girl sustained a head injury from the fall shortly after 5:30 p.m. at 37 Putnam Road, said Captain Donald Treannie, of the Foxborough Fire Department. The severity of her injury was not immediately known, he said. Treannie said both of the girl’s parents were home at the time of the incident but he declined to release their names. It was not clear how the girl fell from the window, he said. No further information was available.