Episode 4 of "Gangnam B-Side" kicks off with Gil-ho marching resolutely towards HYENA, unaware of the peril that awaits. Min, sensing his impending danger, quickly texts him a warning—it's a trap—and attaches a photo of the subway station as proof. But Gil-ho, skeptical of her intentions, disregards the message and enters HYENA with Psyche in tow. Not long after, Min arrives on the scene and calls him, insisting on his caution. He demands evidence, leaving their conversation on speaker as she makes her way to confront Joon-seo.
Inside, they discover an escort adorned with a fraudulent tattoo, realizing Jae-hee isn't present. In a bid to regroup, Min, Dong-woo, and Gil-ho converge at the arcade. Joon-seo, furious at their escape with Gil-ho, vents his frustration by beating Psyche.
At the arcade, Min inquiries about Jung-hwa's whereabouts, prompting Gil-ho to unravel the harrowing tales of Joon-seo's clandestine events where escorts are subjected to whimsical torture, sometimes culminating in murder. In a desperate bid for justice, Gil-ho left Jung-hwa's lifeless body in a public domain, hoping the authorities would initiate an investigation.
Elsewhere, Jae-hee seeks financial assistance from his colleague, Jin-young. Jin-young, bitter over Jung-hwa's destruction of their venture, plans to confront Joon-seo at the Colosseum, brushing aside Jae-hee's cautions. Meanwhile, Tak, enraged by Joon-seo's audacity to eliminate a cop, warns him to concentrate on erasing Jae-hee's incriminating video.
Our protagonists, Min and Dong-woo, plea for Moon's intervention to outwit Tak. Initially hesitant, Moon reluctantly obliges by dispatching the Inspector General to grill Gil-ho. Their probe unearths that Gangnam Trading Company is a smokescreen, Joon-seo is its shadowy overlord, and he's behind the murders of Gang-su and Jung-hwa. Yet, till concrete evidence surfaces, Gil-ho's safety remains paramount. Dong-woo shelters him at Jang-ho's, but the retired cop harbors a deep-seated grudge against Gil-ho, viewing him as a lifelong rival.
Joon-seo is engrossed in the chaotic orchestration of the Colosseum, a bloody spectacle where millionaires wager on a lawless arena of combat. His fury ignites as Psyche dares to interrupt his grim task, yet the broker arrives with Jin-young, who harbors hopes of gaining favors for betraying Jae-hee. However, Joon-seo's mercy is absent; he subjects her to brutal torment until she reveals their hidden refuge. In a fit of rage, Psyche locates Gil-ho's friend, Jung-kwan, and subjects him to a similar fate.
Elsewhere, Gil-ho seeks insights into Dong-woo's feelings for Jae-hee. The detective opens up about Ye-seo's plea and the surprising revelation that the pimp is well-acquainted with her. Jae-hee had once escorted Ye-seo to get a tattoo, a desperate attempt to conceal the scars left by relentless bullying. Their bond blossomed, yet Dong-woo remains skeptical.
Tak rudely terminates Yun's case, citing his demise. As Min prepares to leave with the incriminating file, she witnesses him warmly greeting the Inspector General, who happens to be an old friend. Frustration boils within her; she desires to expose Gangnam Trading Company through the media's scrutinizing gaze. Dong-woo fiercely opposes this, fearing it would shatter the lives of the girls involved, placing them at an impasse.
Tensions escalate when Psyche sends Jae-hee a harrowing image of Jin-young and Jung-kwon, both severely beaten. To finally silence all chaos, she texts Gil-ho, announcing her solitary venture to the Colosseum. Deeply disturbed, he rushes to the event in secrecy, leaving Dong-woo to discover his absence. A frantic call leads to shocking revelations, prompting Dong-woo to consider rushing to the scene. However, Min volunteers to obtain a warrant.
At the Colosseum, Jae-hee, tagging along with a VIP, finds herself trapped within his confines. She defies him, stabbing him and binding him, before sneaking backstage to confront Jin-young, now孤立无援. The brutal fight commences, with Jung-kwon as one of the combatants. Joon-seo announces a staggering reward of 2 billion won for the victor, igniting a betting frenzy among the wealthy spectators.
