Episode 4 of "Spring of Youth" kicks off with Tae-yang defiantly stepping out of his home, bound for the school festival's band performance, despite his father's vehement objections. Meanwhile, Seo Min-cheol reaches out to Cho Sang-heon, who reassuringly confirms that the performance will indeed be a no-show.
At school, Sa Gye and his fellow bandmates bubble with anticipation for their upcoming gig. However, their excitement is swiftly quelled by the arrival of a rowdy mob claiming to be Sa Gye's fans. Their chaotic antics soon turn the venue into a shambles, forcing the cancellation of the performance.
Gyu-ri's sleuthing uncovers the truth: these so-called fans were impostors, prompting Sa Gye to suspect Sang-heon's involvement. Amidst the turmoil, they realize with alarm that Bom is missing. Sa Gye confronts Sang-heon about her whereabouts, but he feigns ignorance. In a twist, Sa Gye locates Bom near the venue, who reveals she witnessed men abducting Tae-yang.
Thanks to Sa Gye's generous donation to the university hospital, he uncovers that Seo Min-cheol has admitted Tae-yang under his care, complete with guards posted outside his VIP room. Sa Gye and his companions rush to the hospital, where they witness a new patient being turned away due to overcrowding. Seizing the moment, the group creates a commotion, insisting that Min-cheol personally sponsor the patient's treatment. While Min-cheol is distracted by the public spectacle, they sneak Tae-yang out and relocate the new patient into the VIP room.
Though grateful, Tae-yang announces his intention to quit the band, fearing for their safety. Yet, his bandmates persuade him otherwise, reinforcing their commitment. In a show of defiance and resilience, they hold their debut concert right on the streets.
Cho Sang-heon's subsequent action involves posting a message claiming that Sa Gye disregards and insults his fans. Consequently, a group of fervent fans converges outside Ja-young's residence to voice their discontent. Their protest escalates into chaos, with intruders storming the premises and creating a scene so severe that Sa Gye is compelled to leap from the rooftop to an adjacent building for refuge.
Over at J&J, Ji-na finds herself entangled in a dilemma when someone uncovers that the chorus of The Crown's latest song plagiarizes an older track. Given the band's rigid schedule, she faces the daunting task of sourcing an entirely new song within just two weeks. Eventually, Ji-na realizes that the melody she borrowed from Bom's song shares similar chords with The Crown's composition, prompting her to incorporate Bom's melody into the new track.
Bom and her bandmates return to their school's band room to find it vandalized, another protest against Sa Gye's actions. Adding insult to injury, Sa Gye arrives with the grim news that their band has been suspended due to a complaint filed by another band, Hanju. Led by Tae-yang, they march to Hanju's band room to confront them, leading to a full-fledged altercation.
The principal lambasts two of Sa Gye's members for assaulting the rival band's members, forcing them to issue a public apology. Soon after, Tae-yang makes the decision to move out of his house officially.
Meanwhile, Kim Ja-young assigns Sa Gye household chores as compensation for the damage wrought by his fans. As he sneaks out later, he discovers a postcard from his mother in the mailbox, hinting at her joyful travels.
Sa Gye then ventures to the store where Bom works, suggesting a date. However, she worries about others discovering their plans. Just then, two girls from Bom's former school in the United States appear at the store, mocking her shift in financial circumstances and her mother's demise. Without hesitation, Sa Gye comes to her defense.
After their departure, Bom reluctantly agrees to embark on a romantic outing. They find themselves standing in a long queue at a bustling bagel shop, their anticipation growing with each passing minute. However, their hearts sink as they watch the last batch of bagels disappear before they can lay claim to one. As fate would have it, once they board a bus, a youthful student, deeply moved by Sa Gye's music, offers his own bagel to Sa Gye, stirring a tender emotion within her.
That serene evening, under a sky adorned with twinkling stars, Bom plays a melody for Sa Gye, expressing her earnest wish for him to lend his voice to the song. He shares the dream that this melody inspired, believing that destiny had guided him to her. As they sip on beer, bathed in the soft glow of moonlight, Sa Gye, unable to resist, leans in to plant a kiss on her lips. To their dismay, Tae-yang catches this tender moment and silently retreats.
The following day, Bom invites Sa Gye to accompany her to visit her mother. She reveals a poignant truth: her mother, having suffered brain death, had generously decided to donate her organs. This revelation triggers a headache for Sa Gye, prompting him to weave together fragments of his dreams with Bom's cherished memories within the confines of their shared home.
Curiosity piqued, he seeks Ja-young's insight into Bom's upbringing. She confirms that Bom indeed grew up in this very house, sharing that her mother tragically passed away in a car accident during a typhoon, her legacy continuing through the donation of her organs, including her eyes. This revelation strikes a chord with Sa Gye, who realizes that Bom's mother expired in the same hospital where he underwent his eye surgery. The dots finally connect in his mind.
His memories also resurface of accusing Sang-heon of a hit-and-run during a party in Episode 1. Driven by this newfound clarity, Sa Gye sets his sights on confronting Sang-heon.
Meanwhile, Bom anxiously awaits Sa Gye's arrival for their planned visit to her mother. Her mind races back to the fateful phone conversation with her mother interrupted by the sound of a nearby car accident. Unable to reach Sa Gye and growing increasingly worried, she reaches out to Tae-yang before succumbing to exhaustion at the conclusion of "Spring of Youth" Episode 4, leaving the future hanging in a delicate balance.
