The fourth episode of "Glass Heart" kicks off with Tenblank making their way to Naoki's secluded countryside abode to dive into the creation of their latest album. Akane grapples with her burgeoning affections for Naoki, contemplating how her heart's capitulation to him was as inevitable as the dawn. Soon after, Kai's arrival injects a dose of reality into the serene atmosphere, informing the group of impending TV appearances and corporate gig requests.
The ensemble convenes to decipher the mystery behind Akane's recent confinement on a boat, with Kai's blatant falsehood about the perpetrator only adding fuel to the already smoldering intrigue. Akane's mind drifts back to her mother's cautionary advice about entangling herself romantically with a fellow bandmate.
As the band converges to brainstorm their upcoming album, Naoki unveils the first track—an original composition by Kazushi, albeit with Naoki's own twist. He's revamped the arrangement to cater to the masses, those who listen through basic headphones and Bluetooth speakers rather than audiophile equipment. Kazushi, though initially taken aback, respectfully proposes to tweak the arrangement further, to which Naoki reluctantly agrees.
Unexpectedly, Yukino bursts into the house, summoned by Naoki for female vocals on one of their tracks. This revelation stirs up even more discord within Kai. Yukino and Naoki's heartfelt, romantic duet practice session only serves to stir Akane's jealousy further, prompting her to unleash her frustration on the drum kit.
Seeking solace and hydration, Akane stumbles upon Kai belting out "Crystalline Echo," her voice resonating with raw talent. Kai confesses that she was originally slated to perform the song alongside Naoki until he deemed it more impactful as a solo effort. Despite this setback, her resolve to propel Tenblank to success remains unshaken.
During dinner, Akane notices Kazushi still tinkering with his new arrangement. She offers him sustenance and listens intently to his work, only to realize he hasn't altered a note. Kazushi, however, explains that Naoki's arrangement harmonizes beautifully with Akane's drumming, whereas his own challenges the status quo. Reflecting on his journey, he admits to cherishing his role in the band and reveals his awareness of Akane's silent admiration for Naoki.
Akane catches sight of Naoki and Yukino serenading by the crackling campfire outdoors and eagerly dashes over to join their harmonious circle. Inspired by a sudden spark of creativity, Kazushi springs back into action, crafting a new musical arrangement that he eagerly shares with the group. Akane recognizes the blend as a fusion of Kazushi's and Naoki's unique styles, and the ensemble applauds in unison—everyone, that is, except Kai. She pulls Naoki aside, insisting they adhere strictly to his original composition and timbre. Kai gripes about Akane getting carried away with the drumming, prompting Naoki to wonder aloud how she even knew drums were aboard. He declares that they should part ways professionally, and Kai storms off in a huff, Sho offering her a ride back to town.
During the journey, Kai finally unburdens her heart. She confesses she spotted Akane lost in the rhythm of the drums, envying the way she became one with the music, much like Naoki does. Sho tries to console her, pointing out that he had always thought her vocals were commendable, but Naoki yearned for a different facet of her talent. This only agitates Kai further, prompting her to demand that Sho pull over, insisting she'll make her own way home.
Back at the house, Yukino informs Akane that she and Naoki have decided to make their relationship exclusive starting the next day. Later, while searching for Naoki, Yukino stumbles upon the sound system and, on a whim, plays a random track. To her astonishment, she hears her own compositions echoing through the speakers.
Meanwhile, Akane tosses and turns, unable to find sleep. She ventures downstairs to find Naoki tinkering with the keyboard and joins him, their fingers dancing in a spontaneous jam session. As the music flows, Akane reaches out and takes his hand, on the verge of confessing something when, unfortunately, Glass Heart Episode 4 concludes abruptly, leaving their story hanging in the air.
