On Elena Ferrante's published trilogy and what comes next

More on Elena Ferrante:

On Elena Ferrante's published trilogy and what comes next:

"Ferrante bewitches with her tiny intricately drawn world,,,'My Brilliant Friend' journeys fearlessly into some of the murkier psychological territory where questions of individual identity are inextricable from circumstance and ever-changing identities."  The Melbourne Review.

After reading 'My Brilliant Friend', 'The Story of a New Name' and 'Those Who Leave and Those who Stay', what several articles mentioned as a trilogy, there will be "an the as yet untittled fourth volume in the series, to be published in September 2015."   So I wonder, will it be a quartet or a saga.