Season 4

John returns to duty after recuperating from back surgery and meets his new partner, Officer Jessica Tang, a tough and disciplined cop with her own set of baggage from working the streets. Lydia, whose new partner is Detective Ruben Robinson, can't seem to escape helping a former CI who seems unable to stay out of danger, Ben, Sammy and Dewey chase an armed man into an elementary school, and Ben bumps heads with an intense street cop whose cynical attitude is affecting their work.

Things are gonna change with Joel Rucker as the new captain. Lydia and Ruben investigate a convenience store murder, and Lydia uses her typical compassionate charm to collect facts on her suspect from an unassuming grandmother. Meanwhile, John and Jessica get to test their physical abilities when they're attacked on the street. Elsewhere, Sammy and Ben ride with a mistake-prone officer, but Ben is the one making mistakes... mistakes that earn him some notoriety.

On Hood Day, a day where gangs seek retribution, LAPD is trying to keep the peace by camping out in neighborhoods prone to violence. Sammy and Ben leave their assigned neighborhood with disastrous results. Meanwhile, the neighborhood grabs a golden opportunity to seek a little retribution of their own against the gang banger who has been terrorizing them. Elsewhere, John and Jessica discover that another group of neighbors will go to any lengths to get a registered sex offender off their streets.

Jessica goes the extra mile to assist a homeless war vet obtain an ID so he can be admitted into a shelter . Meanwhile, Sammy is determined to save a dog after he accidentally shoots it in the leg. Elsewhere, Lydia struggles with a serious personal matter while she and Ruben investigate the murder of a man who sheltered and aided women in need.

John tries to save a suicidal teen in a situation that hits extremely close to home, and Sammy convinces a former addict to be an informant, with devastating consequences. Meanwhile, Jessica gets a visit from someone from her past, and Lydia and Ruben investigate a home invasion that appears to have been staged to cover up a murder.

Lydia dons a uniform and hits the streets when the shift is short a supervisor and she must fill in. Meanwhile, John and Jessica are stuck with a film crew tagging along on their calls. Elsewhere, Ben infuriates Sammy when he implies Sammy planted evidence.

Jessica and John exchange angry words after she lies to Internal Affairs about the facts surrounding her shooting of a teen with a toy. Meanwhile, Sammy continues to hold a grudge with Ben and Lydia continues to dodge questions about her condition.

Lydia and Ruben investigate the death of a young nanny, but first they have to find the kids who were in her care. Ben tries to intervene in the life of a prostitute with a daughter, but his efforts only make things worse. Tang gets an update about the boy she shot, but John still wants her to come clean about what really happened.

Ben finds himself heading down a slippery slope as he attempts to help the daughter of a prostitute, putting him in hot water with the woman's pimp. Lydia and Ruben investigate the murder of a gang-banger who may have been killed over a graffiti tag. Tang faces more questions about her shooting of an innocent bystander, which could determine whether she gets promoted to sergeant.

It's the last day on patrol for Tang, as she prepares to move to another precinct as sergeant. But first she wants to tie up some loose ends on the beat. Lydia is pressured to make a decision about the extent of her police duties during the rest of her pregnancy. With Sammy recovering from injuries, Ben and Dewey are partnered together. But Ben is bent on tracking down the prostitute he's been trying to help and then getting her and her daughter away from the abusive pimp controlling their lives.