Midway through the season (Episode 8), Antonio leaves the Intelligence Unit to join the State’s Attorney’s office as a chief investigator, a move designed to lead into the spin-off Chicago Justice .
Introduced in Episode 21 as a Detective from Robbery-Homicide, Upton eventually joins Intelligence, filling the vacancy left by Lindsay.
Season 4 marked the end of an era for two of the show's original leads and introduced a future mainstay: Chicago P.D. - Season 4
A crossover with Chicago Fire (Season 5, Ep. 9) where Kelly Severide is suspected of vehicular manslaughter.
The season's narrative is framed by accountability and the personal cost of life in the Intelligence Unit: Midway through the season (Episode 8), Antonio leaves
In a devastating multi-episode arc, Alvin Olinsky (Elias Koteas) loses his daughter, Lexi, in a warehouse fire, pushing him to a grief-stricken breaking point.
The season begins with Commander Emma Crowley (Barbara Eve Harris) investigating Voight and Lindsay for the murder of Justin Voight’s killer, a dark secret that strains their relationship. 9) where Kelly Severide is suspected of vehicular
In "All Cylinders Firing," Desk Sergeant Trudy Platt (Amy Morton) is brutally attacked outside her father's home, leading to a personal and harrowing investigation into her past.