Season 4 featured several "One Chicago" crossovers that connected the unit to Chicago Fire and Chicago Med :
A crossover with Chicago Fire (Season 5, Ep. 9) where Kelly Severide is suspected of vehicular manslaughter.
Briefly joined the unit as a recurring detective while Adam Ruzek (Patrick John Flueger) was on an undercover assignment. Major Plot Arcs Chicago P.D. - Season 4
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.
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. Season 4 featured several "One Chicago" crossovers that
Introduced in Episode 21 as a Detective from Robbery-Homicide, Upton eventually joins Intelligence, filling the vacancy left by Lindsay.
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. Major Plot Arcs The season begins with Commander
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 .