According to Der Spiegel, Eva Gabrielsson now says there will be no fourth volume joining the Millennium Trio.
“Stieg is dead. There are three books. We should leave it at that.”
Of course, it isn’t up to Gabrielsson to decide. It is up to the rights-holders, who are not Gabrielsson but Erland and Joakim Larsson. If they decide there will be a fourth, or for that matter fifth and nth volume, that’s what will happen.