Aaron Douglas and Ryan Robbins will be recurring this season, and Ben Cotton will make an appearance in at least one episode.
Here is the setup, from Alan Sepinwall's review:
The new season picks up a little over a year after the last one concluded with the election, the arrest of Aunt Terry, and Mireille Enos' over-invested cop Sarah Linden walking away from Holder and the job. Holder is climbing the ladder at the Seattle Pd with the advice of new partner Carl Reddick (Gregg Henry) when he catches the murder of a teenage prostitute who was killed in a manner very similar to a case that Linden worked years before with her old partner James Skinner (Elias Koteas). Linden and Skinner arrested their victim's husband,