As of March of this year, so far the only thing known is the screenwriter David Magee (Life of Pi, Finding Neverland, Miss Pettigrew Lives For a Day). And it is tentatively scheduled for an initial release date of sometime in 2016. Although dates are subject to change obviously, because in this case nothing in production has even started yet.
The film is in script stages and that goes through many stages as well. So the film may take a while. Especially since this film was supposed to have come out many years ago.
The Chronicles of Narnia: The Silver Chair is based on the fourth book published in C.S. Lewis’ famed fantasy series. The first three Narnia films combined have grossed over $1.6 billion at the worldwide box office. The book, which features the return of The Voyage of the Dawn Treader's Eustace Scrubb (played by Will Poulter in the 2010 adaotation), is described as follows:
Through dangers untold and caverns deep and dark, a noble band of friends is sent to rescue a prince held captive. But their mission to Underland brings them face-to-face with an evil more beautiful and more deadly than they ever expected.
Eustace Scrubb, Jill Pole, and Puddleglum set off on a journey to the land north of Narnia in order to rescue King Caspian's stolen son, Rilian.
Eustace Scrubb, Jill Pole, and Puddleglum set off on a journey to the land north of Narnia in order to rescue King Caspian's stolen son, Rilian.