He has said that he considers the trilogy complete and that he has no plans to add to it. He has also said that he believes that the story of Lyra and Will is finished and that there is nothing more to say.
However, he has also said that he is open to the possibility of writing a prequel or sequel to the trilogy in the future, but he has not announced any plans to do so.
So, while it's unlikely, a fourth book is not impossible.