In the suspenseful fourth episode of the Korean drama "Connection," the enigma of a concealed truth and the shadows of a drug conspiracy deepen. Oh Yun-jin, entangled in the mystery, realizes that the perpetrator who brought Lee Minguk's case to light is aware of her, Jang Jae-kyeong, and Lee Minguk's shared affiliation with the same club. Back at the police station, Jang Jae-kyeong battles his drug addiction, only to succumb unwittingly, prompting him to delete the surveillance footage that could expose his vulnerability. In an unexpected twist, he discovers that Kim Chang-soo has also accessed the surveillance footage that day.
At the police station, Jang Jae-kyeong reports to the chief that Lee Minguk was hidden and then deliberately unearthed, suggesting different individuals are involved, and the person who threw the Molotov cocktail appears to be erasing incriminating evidence. Meanwhile, Oh Yun-jin, aiming to write about the case, withholds Lee Minguk's intention to expose a scandal, a self-preservation tactic to avoid drawing the killer's attention. In her quest to protect Jang Jae-kyeong and her neighbor's child, she also considers abandoning her report on the new drug.
Jang Jae-kyeong is suspicious of Kim Chang-soo's motives for secretly reviewing the police station's surveillance footage. After all, it was Jang Jae-kyeong who informed Kim Chang-soo of Lee Minguk's location, and soon after, a Molotov cocktail was thrown with alarming precision. During the interrogation of Kang Jin-wook, a drug user, they learn that the source of the drugs is a man with a distinctive tattoo on his finger, "Lee Geun-ho."
On another front, Kong Jin-wook discusses obtaining the goods after proactively contacting "Doctor," noting the use of voice modulation and a change in the rules of communication that did not elicit any anger from the other party.
Jang Jae-kyeong begins to probe Kim Chang-soo, who had deliberately reviewed the hospital's surveillance footage and left hastily near Lee Minguk's home without asking further questions. Kim Chang-soo's calm and compliant demeanor raises Jang Jae-kyeong's suspicions that he might be helping to cover up the incident or even "bought off to conceal the truth."
Won Jong-soo, upon seeing the arson report on Lee Minguk's home, confronts Park Jun Seo, questioning why Lee Minguk's body was found there. Park Jun Seo suggests that Lee Minguk's death is a blessing in disguise, as it's easier to handle than his sudden reappearance alive. Won Jong-soo's friends are now pushing for him to become the chairman of the conglomerate to ensure the smooth progression of the residential development project. Park Jun Seo claims to introduce potential investors, but Won Jong-soo dismisses him with contempt.
Jang Jae-kyeong investigates the surveillance footage around the site of Park Joon-woo's fall and finds nothing suspicious. Oh Yun-jin receives an anonymous tip about a medical error at the hospital that led to the death of "Park Bok-ri," but the deceased's son is reluctant to discuss the matter. Ji-yeon suddenly asksHeo Ju-song to meet, expressing her belief that her husband intended his death to be a lure for Jang Jae-kyeong and Oh Yun-jin to investigate, and asks for his help in a private inquiry.
Kong Jin-wook arrives at the location designated by "Doctor," where he is introduced to new trading methods and principles for picking up goods. Unbeknownst to others, Park Jun Seo and Ji-yeon are having an affair. Park Jun Seo threatens Ji-yeon to reveal a password, but she counters by demanding a billion won in exchange for the password and her silence about their affair.
Oh Yun-jin spots Park Jun Seo entering Ji-yeon's home and follows Ji-yeon, discovering their secret rendezvous. Jang Jae-kyeong visits the construction site where Park Joon-woo fell, seeking answers to why he chose this location. He discovers a small, obscure hole known only to a few workers, which leads to a nearby rice cake shop with surveillance footage. Jang Jae-kyeong sees Park Joon-woo arriving at the site by taxi half an hour early and another car arriving later. He also learns that Park Joon-woo had requested surveillance footage six months prior and even gifted the shop owner a health product from Jin-kyung Pharmaceutical. Unfortunately, all of this was witnessed by Jung Minho, leading to the murder of the rice cake shop owner to eliminate evidence and silence him. In the end, Jang Jae-kyeong only manages to retrieve part of the footage, missing the crucial scene of Jung Minho transporting Oh Ji-hyun.