In Spring of Youth Episode 4, deepening emotional connections and buried secrets come to a boil. Tae-yang walks out of his house to join his band’s school festival performance, ignoring his father’s harsh warnings. Meanwhile, Seo Min-cheol is confident the show won’t happen, thanks to some behind-the-scenes interference. Just as Sa Gye and the others get ready to play, chaos erupts — a group of fake fans wrecks the venue, ending the show before it starts.

Gyu-ri quickly proves they aren’t real fans, making Sa Gye suspect Sang-heon’s involvement. That’s when they realize Bom is missing. Sa Gye finds her nearby, shaken — she saw Tae-yang being taken away by strange men. Following the clue, Sa Gye learns that Min-cheol has locked Tae-yang up in a hospital VIP room, guarded on all sides. Sa Gye uses a clever distraction — posing as a sponsor for a patient in need — to cause a public scene and free Tae-yang, sneaking him out while the hospital handles the PR mess. Grateful, Tae-yang still insists on quitting the band to protect the others, but his friends convince him otherwise. The band makes their real debut, performing on the street.

Sa Gye’s troubles grow beyond the stage

But Sang-heon doesn’t stop. He posts that Sa Gye insulted his fans, triggering a protest outside Ja-young’s home. Fans barge in, forcing Sa Gye to flee by leaping across rooftops. Meanwhile at J&J Entertainment, Ji-na is in a bind — The Crown’s new song has a plagiarized chorus. With only two weeks to fix it, she recalls Bom’s melody and uses it as a base for a replacement, likely without crediting her.

Back at school, the band finds their practice room trashed in retaliation. Worse, they’re suspended due to a complaint by the rival band, Hanju. The tension explodes into a fight, and the school demands a public apology from Sa Gye’s group. Tae-yang also officially cuts ties with his family, moving out for good.

A bagel date and painful memories

Sa Gye, now staying at Ja-young’s, is forced to do chores as payback for his fans’ destruction. On his way out, he finds a postcard from his mom, who seems to be traveling happily. He then visits Bom’s workplace and asks her on a date. She hesitates, but after two girls from her U.S. school mock her for being poor and losing her mom, Sa Gye steps in and defends her — prompting Bom to agree.

Their date ends with a touching moment: a schoolgirl recognizes Sa Gye and gifts him her last bagel because his music helped her through hard times. Later, under the moonlight, Bom plays him the melody she wrote and asks him to sing it. Sa Gye reveals he dreamt about this very song — a sign that fate brought them together. They kiss, unaware that Tae-yang sees everything and quietly walks away.

A haunting truth emerges

Bom invites Sa Gye to visit her late mother with her the next day and opens up: her mother was brain-dead after a car accident and she donated her organs. That confession triggers something in Sa Gye. He connects it to the dream he had — and suddenly his past starts clicking into place.

Asking Ja-young more questions, he learns that Bom’s mother died during a typhoon-related car accident — the same time Sa Gye had his eye surgery at that hospital. It dawns on him: Bom’s mother was likely his donor. A memory of accusing Sang-heon of a hit-and-run during the party resurfaces. He storms off to confront him.

Meanwhile, Bom waits alone, worried as Sa Gye never shows. A car crash nearby triggers her trauma — she remembers talking to her mom just before the fatal accident. Calling Tae-yang for help, Bom collapses from the overwhelming weight of it all, closing Spring of Youth Episode 4 on a haunting note of fate, trauma, and unraveling truths.