The fourth episode of "Hyper Knife" kicks off with Se-ok aboard a bus, bound for the comfort of her home. She alerts the hospital of her inability to attend work, prompting Ha U-yeong to inform Deok-hee that Se-ok has headed back to fetch some winter attire. The duo subsequently visits Se-ok's abode, where she is taken aback by their unexpected arrival. Equally surprising is the sight of Se-ok bearing a few bruises on her face.
With a casual remark that she slipped on ice, Se-ok attempts to brush it off, but Deok-hee isn't easily fooled. He dispatches U-yeong to fetch some medication while questioning Se-ok intently. During this interrogation, Deok-hee discovers a deceased male body in the shed, prompting a confession from Se-ok. She reveals that the man had assaulted her on her way home, leading to a fierce struggle where she eventually stabbed him with her knife.
Deok-hee inquiries calmly if she needs assistance in disposing of the body, and Se-ok confidently asserts that she doesn't. As he departs, Se-ok's face beams with relief, interpreting his offer as a gesture of support.
Later, at the newly uncovered crime scene near the Buddhist temple, Se-ok and Young-joo pause to observe the commotion. Se-ok seems to revel in the excitement, her enjoyment waning only when she spots Choi Deok-hee amidst the flurry of activity. U-yeong arrives at the scene to return Deok-hee's belongings, and together, they notice Se-ok engaging playfully with a police dog. She boasts to Detective Lee about her Rottweilers, revealing a certain affinity for fierce creatures.
In separate conversations, Se-ok and Deok-hee exchange words. Se-ok presumes Deok-hee's presence at the crime scene is an attempt to intimidate her into undergoing his surgery. She vows to expose him for Kim Myeong-jin's murder, her resolve unwavering. As she exits, Inspector Yeom comments to Deok-hee, hinting at Se-ok's forgetfulness regarding the harm she once inflicted upon him, adding an air of intrigue to their already complex dynamic.
Some time later, Se-ok crosses paths with Kim Ki-young, who reveals that he has received a tip claiming sighting of his father at a now-defunct mental hospital. Intrigued and eager for answers, Se-ok expresses his desire to investigate the place. Meanwhile, in a different setting, Deok-hee finds herself dining once again with Inspector Yeom and Detective Lee. They unfold their investigation by presenting her with photographs of forensic evidence from the morgue, including an intriguing image of some hair.
Se-ok and Ki-young subsequently track down the individual who passed on the tip. This man, a former employee of the hospital, reveals that he once witnessed Kim Myeong-jin under the alias Lee Seok-gu in room 401. As he guides them through the eerie corridors of the abandoned hospital, Ki-young begins to feel a growing sense of unease. The ex-employee further mentions encountering a grey-haired visitor to Lee Seok-gu's room, prompting Se-ok and Ki-young to cast doubts on his narrative. To verify his story, the ex-employee contacts a colleague and requests the forwarding of a photograph taken during a press event with the patients.
Elsewhere, Mrs. Ra confronts Deok-hee with pictures depicting Se-ok in the company of Ki-young, revealing her surreptitious surveillance. Deok-hee urges her to allow Se-ok the agency to uncover the truth on her own terms. Later, Deok-hee convenes with her friend Song Eun-chae, who doubles as the physician conducting the autopsy. Their meal is interrupted when Song Eun-chae calls Deok-hee to inform him that the hair discovered in the murder case is indeed Rottweiler fur, aligning with his suspicions. Ironically, this revelation transpires while Deok-hee finds herself amidst the serene ambiance of an art gallery.
A flashback draws us back to a moment when Deok-hee discovers Se-ok slumbering in a laboratory. He gently chides her for her surgical addiction, to which Se-ok candidly admits that neither food nor sleep hold the same allure for her. With concern, he exacts a promise from her to prioritize her well-being, urging her to eat and sleep adequately.
Currently, Ki-young's anger is directed at the hospital employee, as he is yet to receive the photograph. The man's palm bears an injury, and they come to realize that he has taken Kim Myeong-jin's ring. This discovery ignites a blaze of fury within Ki-young, prompting him to chase the man relentlessly until they both plunge off the edge of a steep cliff. Tragically, the hospital employee loses his life, while Ki-young finds himself hooked to machines in the hospital.
Clad still in her blood-stained attire, Se-ok bursts into Deok-hee's office at the hospital, presenting him with the ring. He scoffs at her, calling her pathetic, to which she retorts fiercely that she would have killed Myeong-jin for him. She insists on knowing whether he evicted her out of shame for being caught in the act. Deok-hee remains silent, offering no response.
As Deok-hee speeds towards his home, his reckless driving is captured by a traffic camera. Upon the departure of his house cook, Deok-hee heads to a building where a sleek, white car awaits. A flashback ensues, revealing Deok-hee and Myeong-jin entering the same car. Myeong-jin takes the driver's seat and notices something sticky on the door handle. Deok-hee offers him a wet wipe to clean it off. The narrative then jumps to a tense scene where Deok-hee holds a knife to Myeong-jin's throat as he makes a final phone call to his son.
In the present, Deok-hee drives the same car to pick up Detective Lee. Lee inquires about the purpose of their meeting. However, like Myeong-jin before him, Lee touches the sticky substance and uses Deok-hee's wet wipe, only to find himself unconscious in the car by the end of Hyper Knife Episode 4, leaving a cliffhanger that grips the viewer's attention.
