Sotheby’s experts uncovered the secret this month as they were examining “Orange Sports Figure,’’ which goes on sale Wednesday. The vibrant image of an abstract crowned figure is estimated to be worth between 3 million pounds and 4 million pounds ($4.7 million and $6.3 million).                                                                                                                                                                                                                  Basquiat, a graffiti artist who became a 1980s art star, signed relatively few of his canvasses. But Sotheby’s said ultraviolet light revealed the artist’s name and the date 1982 beneath the work’s layers of acrylic and spray paint.