Episode 4 of Concrete Market commences with Hee-ro suggesting they take control of room 902. In response, Tae-jin shares a bit of his past, detailing what happened after the earthquake. He recounts living in a basement with a group of survivors, where a fight broke out over leftover food. Tae-jin was stabbed in the leg and couldn't move. When they exhausted their food, they had to move, but Tae-jin was still injured. The gang leader initiated a poll on whether to leave him behind, and they all voted to leave him. Certain of his imminent demise in the basement, Tae-jin was saved by Chairman Park and Lee Mi-seon, who nursed him back to health. Mi-seon works on the eighth floor.
Hee-ro realizes that Tae-jin will most likely never betray Chairman Park, so she devises a different revenge plan. She needs to get close to Chairman Park herself and knows that the Chairman controls all the medicine on the market. She decides to use it.
Hee-ro locates the people dealing with medicine and requests heart medicine. Later, a problem arises in the market when the water stops flowing due to clogged pipes. The Chairman asks if any of the residents can solve the pipe problem, and Hee-ro volunteers. She promises to resolve the issue within three days if the Chairman provides her with the blueprints of the building’s sewerage system.
Chairman Park invites Hee-ro to his room for a chat. He prepares a delicious meal, which Hee-ro barely touches. It turns out that he has developed feelings for Hee-ro. He recalls Se-jeong, who was the same age as Hee-ro, and he liked her. Chairman Park offers Hee-ro a job with him. He shares his backstory from before the earthquake, when he was a salesperson for a pharmaceutical company.
Although he exerted tremendous effort in his job, he led a modest lifestyle. The only ones who profited were the company owners and their associates. Now, he reigns supreme in the concrete market, and Hee-ro can assist if she chooses to befriend him. The news of Hee-ro's search for a heart medicine had already reached Chairman Park's ears. He uses this to coerce Hee-ro, offering her a tablet for every piece of information she provides. Hee-ro is elated that her plan has materialized.
She rushes back to the room to share the news with Se-hee, but she is nowhere to be found. Before she could venture out to look for her, she runs into Tae-jin outside the door. He pulls her aside to speak. Meanwhile, Cheol-min had earlier noticed Se-hee and thought she resembled Se-jeong. He puzzled over it, trying to figure out who she was.
In the meantime, Se-hee is outside, taking notes as she listens to gossip about Chairman Park, the market, and Se-jeong. She is unaware that Cheol-min is observing her and has noticed that she is wearing clothes similar to Se-jeong's. Cheol-min sees her from afar as she goes back to the room.
Elsewhere, Tae-jin intends to remove Mi-seon from the eighth floor and needs Hee-ro's assistance. Hee-ro points out that it will be challenging since Cheol-min manages the eighth floor and is unlikely to help them. Tae-jin employs a power play to force Hee-ro's cooperation, reminding her that he is the boss and she must obey his orders and come up with a solution.
At the episode's conclusion, Hee-ro returns to the room and discovers that Se-hee has returned. She is angered that she had left knowing it was risky and used their food to trade for a music box. Hee-ro desires to honor Se-jeong's final words, so Se-hee promises not to leave again. Just then, someone pounds on their door. Hee-ro goes to investigate and discovers it is Cheol-min.
