As we left season 3, Ed’s life was in danger, being shot at. Team One races to find Ed’s shooter, while his wife has problems delivering their baby. The culprit turns out to be a man married to a military wife under duress from drug groups based in Toronto, who plan on smuggling drugs through military channels from Afghanistan. Team One pulls off a risky move to capture the culprit. Afterwards, Dr. Toth (Victor Garber) clears for duty Team One, but keeps it under probation. As he recovers from his wounds (with the vest having saved his life), Ed meets up with his wife, son & new baby. In the next episode Team One has to protect a cop who incidentally shot a teenager, but was cleared of all charges. The boy’s father incites a mob, demanding justice for his son’s death. But the event soon turns to violence and a desire for vengeance. The team goes after a corporate robber who streams his robberies on the Internet and has thousands of followers. However, the robber plans on making a “grand finale,” unaware that he’s being duped by the stepson of a philanthropist, posing as a teenage girl, into assassinating his neglectful father.

The SRU is called when a mother and daughter are suddenly abducted, and the only clue to the abduction could expose a dark family secret. During a drug bust, one member of Team One struggles with a secret that could jeopardize the member’s career. The situation worsens when the undercover officer in the drug cartel becomes too emotionally involved with the cartel leader’s girlfriend and refuses help from Team One. A new recruit, Raf (Clé Bennett), replaces Wordy on Valentine’s Day, traditionally one of the toughest days of the year for hostage and suicide situations. Team One splits up to defuse three cases: a suicidal widower; a mom holding her daughter’s manager of a strip club hostage; and a recently fired gunman who is obsessively infatuated with his boss. Greg directs the saving of the last victim, who later hints at a romantic interest in him. Spike must leave his dying father’s bedside to help Team One defuse its largest bomb threat yet. Two smaller bombs go off in a building, trapping Spike, Raf, Sam and six other civilians underground. Spike then discovers a complex chemical bomb designed to produce a bigger explosion. The bomber is one of the civilians trapped underground, determined to get revenge after the firm he worked for framed him for selling out company secrets when their stock dropped. Spike must deactivate the bomb before it explodes. Team One is called when a group of armed hijackers forces a passenger jet plane to land. As Parker tries to negotiate with the increasingly desperate criminals, the rest of Team One faces an almost impossible task—how to storm the plane without anyone getting killed. With the help of brave passenger Holly McCord (Melanie Scrofano), the team races to find a solution before hijacker Sandra Moore (Joanne Boland) carries out her deadly threats.

Team One responds to a panicked 911 call: a masked gunman is threatening a group of teenagers in the Don Valley ravine and has abducted Joe Stanick. But it turns out this gunman was no stranger to the youths. Joe was the author of a viral video prank that “outed” a fellow student, Ryan Bell, triggering humiliation at school, and an explosive confrontation with his parents. Now, Ryan’s engaged in a desperate and violent journey, hoping to win back his parents’ acceptance. Ed faces a sensitive suicide intervention that hits a bit too close to home… and Parker, inspired by Ryan’s struggle, gathers his own courage to speak the truth to the woman he loves. When a ransom drop goes wrong, Team One is drawn into the dangerous world of “express kidnapping” and the nightmare of a woman haunted by the brutal experience of captivity. Meanwhile, Greg finds out about Sam and Jules’ secretly renewed relationship. A gambling-addict mother (Camille Sullivan) under duress from a loan shark who owes money to the local crime boss, Harvey Micks, is forced to play her life away. After a shooting on Donna Sabine’s wedding, Team One speculates that the attacks are retaliation made by the local crime family, the Logans, on Donna’s former undercover team. Instead, the Logans are targeting the team’s family members after their names are accidentally leaked by Donna’s former partner, who is then forced by the Logan matriarch to try to kill Donna’s newly-wedded husband.

A Chinatown gang called the Golden Dragons kidnaps a widower to force his daughter to give up the names of the various store owners building up evidence on the gang’s extortion rackets. Team One races to Fletcher Stadium, where a knife-wielding robber has taken a hostage. The situation turns bloody when a security guard tries to be a hero. The guard, Gill Collins (Aaron Douglas), is an ex-cop who applied for SRU who had set up the robbery in an attempt to make himself a hero. With that plan failed, he takes Greg hostage, blaming him for his fall from grace. Spike responds to a distress call from Sam’s sister and suddenly finds himself held at gunpoint by her criminal ex-boyfriend. Meanwhile, Team One responds to shots fired at a metro depot, where two men have broken in to rob it. eam One defuses an explosive conflict at a high-security psychiatric hospital, only to find that a young patient, Charlie (guest star Rossif Sutherland) has escaped, bent on confronting his former best friend about a hidden secret. As is turns out, Charlie, who was hospitalized for incidentally pushing a classmate to his death for bullying him, took the fall for his friend, who was the real culprit. And now his friend is poised to move out of town without clearing his name. A laboratory filled with deadly anthrax samples is broken into and hostages are taken and an explosion traps Jules.Meanwhile, Parker is asked to resign or face suspension for keeping Sam and Jules’ relationship a secret.

During the season four finale, Team One races to a hospital when Fire Captain Simon Griggs (guest star Dylan Neal) interrupts the attempted murder of a badly burned firefighter. When they corner the suspect, the team learns he’s behind a series of deadly fires in the neighborhood. Meanwhile Fire Captain Griggs is forced to make a horrific choice—one that pushes him to the edge of revenge and suicide. Parker, wrestling with his own decision about his future on Team One, must convince this broken man to live. As the season ends, we see the team with their families at a family picnic, where Parker comes to a decision about his continuing with the team and decides to stay.