A friend of the late author Stieg Larsson claims that he's read the unpublished manuscript for the fourth book of the Millennium series, and shared plot details with a Swedish newspaper. From the Hollywood Reporter:
Larsson confidant Kurdo Baksi told Swedish daily paper the Expressen that Camilla, the estranged twin sister of Millennium's goth hacker heroine Lisbeth Salander, plays a major role in the fourth book. Camilla was seen only briefly in the second Millennium novel, The Girl Who Played With Fire.
Baksi is quoted in Expressen as saying the fourth novel is set on the remote Banks Island in Canada and the further deepens the relationship between Salander and investigative journalist Mikael Blomqvist. Baksi, whose book My friend Stieg Larsson was a best-seller last year, added that before he died Larsson completed a detailed introduction and a finale to the fourth book.
While this news may fuel more fan fiction, don't hold your breath for the actual novel. According to previous reports, the text was actually meant to be the fifth, not the fourth book in the series, and the family has said they won't let the novel be published.
Details From Fourth ‘Dragon Tattoo' Book Revealed [Hollywood Reporter]