Episode 4 of "In Your Radiant Season" unfolds with Chan's psyche jolted by the glare of a passing car's headlights, prompting a recollection of a new sound—a tinkling reminiscent of tiny metal fragments or ice crystals—linked to the explosion but eluding immediate identification. He visits his psychologist, who suggests that encountering Ha-ran has reawakened a fragment of his missing memories. The therapist advises maintaining some distance to safeguard himself, but Chan remains resolute in his quest to aid Ha-ran.
One evening after work, as Ha-ran is about to leave the office, Chan unexpectedly appears with a taxi, inviting her on a picnic by the tranquil Hangang River. He calls it a preview, as he's about to embark on a two-week fieldwork assignment in Gyeongju. A nearby couple's mention of the fireworks at Jamsugyo Bridge triggers Ha-ran's memories of walking by that spot when she heard the instrumental tune shared with Kang Hyeok-chan—the same incident where Chan encountered her post-accident, restoring all his memories. Presently, Chan notices her expression darken and offers their fried chicken to another group, instead opting for two bicycles to ride across the Jamsugyo Bridge for some grilled meat. Ha-ran finds herself smiling despite the distressing memories.
The next morning, Ha-dam encounters Han So-ra at school, attempting to procure You-gyum's contact information. At work, Na-na arrives at a conference with incorrect materials and requests Ha-ran to fetch the correct ones promptly. She asks Chan for a ride but is disoriented by a car's honk, causing him to swerve and halt abruptly. Ha-ran is shaken by the incident, but he assures her everything is alright and sends her off in a cab.
Later, they meet again, and Chan explains it was his left ear that caused the disturbance—a reminder of Ben's injury—and apologizes for alarming Ha-ran. She counters that his "free trial" is already aiding her recovery, enabling her to smile at Jamsugyo despite the sadness associated with the memories. They discuss their gradual getting-to-know-you process, with Ha-ran revealing her aversion to oysters. In turn, she invites Chan to share a secret with her someday.
That evening, You-gyum arrives at the doorstep of Ha-dam's family, his eyes widening in recognition as he realizes he's seen Na-na before. It was on the road, where she was disoriented and he'd come to her aid. Now, she's whispering to him, pleading with him not to reveal the truth to Ha-dam. After an awkward pause, You-gyum takes his leave, promising to return later.
Ha-dam is livid, especially since her sisters haven't arrived on time. Meanwhile, Tae-suk keeps butting into Ha-young's plans to meet Cha-min. At school, You-gyum tells Ha-dam he can't hang out after school, and she soon realizes he's lying. Later, she sees him meeting someone else, but it turns out to be Ha-young and Ha-ran! They wanted to apologize to him.
However, a flashback reveals that You-gyum had lied earlier because he was meeting Na-na. She told him that she has dementia and asked him not to tell her granddaughters the truth. Elsewhere, Tae-suk confronts Cha-min about juggling different women. He destroys his phone and orders him to break up with the women.
Ha-ran and her team decide to go to Gyeongju as well. Ha-ran decides to leave earlier and go with Chan, offering to drive. He knows she's doing it out of worry and appreciates it. Suddenly, it starts raining heavily and the windshield wiper stops working. Ha-ran can't see anything, and Chan looks out the window and guides her to a rest stop. Chan calls the rental company, whose people arrive and proclaim that the car will only be replaced in 4-5 hours.
Chan calls his grandmother for help, who lives nearby and is delighted to meet him after so long. She takes them home, where she runs a pottery studio. She tells Ha-ran how his mother was an artist but his father hated it. Chan's mother wanted to go abroad to study but pushed herself too hard to come back quickly to Chan, eventually getting sick and passing away. That's why his father hated art and made Chan go to the US.
Back at the office, Ha-young learns that Cha-min is now handing out wedding invitations. She confronts him and delivers a well-placed kick. On her way back, she comes across Tae-suk and learns that his car is being repaired. She puts the dots together with what Cha-min told her and asks Tae-suk if he was the one who spoke to Cha-min. He avoids answering and scurries away.
At Chan's grandmother's house, she asks Chan if he likes Ha-ran. He says she's someone he shouldn't have feelings for, which confuses his grandmother. Afterwards, Chan and Ha-ran get their car back and bid his grandmother goodbye. Suddenly, Ha-ran asks Chan why she's someone he shouldn't like, confessing that she overheard his conversation with his grandmother. She says she's curious about why he put it like that. He then asks her if she's okay with him liking her.
