BROOKLYN - Family members say a father of four was gunned down today inside his Crown Heights apartment building. Relatives say Shamell Williams, 34, was fatally shot this afternoon at 320 Empire Blvd. They say he was under house arrest at the time, but are unsure of a possible motive, noting that he had little money. Williams' ex-girlfriend, who is the mother of his four children, says he was an excellent father and a great friend.