Season 4 opens with John returning to work after recuperating from back surgery and meeting his no-nonsense new partner, Jessica Tang (Lucy Liu). Elsewhere, Lydia also gets a new partner; an armed man is pursued into an elementary school; and Ben tangles with a cynical street cop.
Tang's and Cooper's physical vulnerabilities are sorely tested when they're attacked; Lydia and Ruben put their focus on a murder at a convenience store; Ben and Sammy team with a bumbling officer; and Joel Rucker assumes the captain post.
Violence explodes on Hood Day when Sammy leaves a gang territory unattended; Ben is haunted by his recent mistakes; Ruben prepares for his daughter's birthday celebration; and parts of Tang's past surface.
Tang's past comes to the surface when she offers help to a homeless veteran. Elsewhere, Sammy cares for a dog after accidentally shooting it; Lydia and Ruben investigate the murder of a good Samaritan; Ben and Lydia brave monumental life changes.
A case turns personal for John when he tries to save a suicidal teen; Sammy's attempt to transform a former drug addict into an informant comes back to haunt him; a person from Tang's past arrives; and Lydia and Ruben scrutinize a home invasion.
A young man set to testify in one of Sammy's cases is killed; John and Tang enter the reel world when they serve and protect a documentary-film crew; and Lydia's body is tested when she works as a shift supervisor.
Tang's bad day turns worse when she's served legal papers and is involved in a shooting. Meanwhile, tension flares up between Sammy and Ben over the possible planting of drug evidence; and Lydia makes a big decision about her pregnancy.
Lydia and Ruben probe the death of a nanny. In an alarming investigative snag, the children who were in her care have gone missing. Meanwhile, Ben's efforts to help a prostitute with a daughter backfire; and John corners Tang about coming clean about a past shooting.
Ben tries to help the daughter of a prostitute, only to draw the wrath of the woman's pimp. Meanwhile, Lydia and Ruben snoop into the murder of a gang member; and Tang faces the mounting consequences of shooting an innocent bystander.
Tang reports for her last day on patrol before moving on to another precinct in the Season 4 finale. Meanwhile, Lydia is pressed to make a decision regarding her pregnancy; Sammy recuperates from injuries; and Ben searches for the woman he's been trying to help.