In the show's winter finale, Gina Linetti gets hit by a bus in the final scene. It is later announced that Chelsea Peretti is expecting her first child although Peretti has confirmed that her pregnancy and Linetti's accident is not related
On February 7, 2017, it was reported that Nathan Fillion would guest star as an actor playing a detective who wants to help during an investigation in the set of the show he stars in, Serve and Protect. Greg Germann would also guest star as the show's executive producer. 
On February 14, 2017, TVLine reported that Gina Gershon had signed on for a multi-episode arc playing Sergeant Melanie Hawkins, a notoriously bad-ass cop who runs the NYPD's most elite task-force.
On March 3, 2017, it was reported that Kyle Bornheimer will reprise his role as Teddy Wells, now an NYPD auditor who's evaluating precincts to help determine which precincts should be shut down.
On March 10, 2017, Melissa Fumero and Rick Page took to Instagram to post about the Season Four finale.
On April 3, 2017, L. Scott Caldwell was announced to play Laverne, Captain Holt's mother.
On April 13, 2017, it was revealed that Ryan Phillippe would play Milton, Gina's love interest although their pairing would upset Charles.
Now fugitives, not only are Jake and Holt being hunted by Figgis but now also Florida Police. The 99 heads to Florida to help Jake and Holt. For disobeying their captain, the squad gets assigned to the night shift.