Delta to Gamma no Rigakubu Note Vol. 4 Reveals Cover Ahead of October 9 Release

The fourth Delta to Gamma no Rigakubu Note light novel, Natsuzora ni, Michi o Sagashite. (夏空に、未知をさがして。), has revealed its cover ahead of its October 9, 2026 release in Japan.

Written by Takuma Sakai and illustrated by Asagi Tohsaka, the new volume takes Izuta and his group beyond Tsunanagai for a summer trip to Nagasaki. A second thread reaches two years into the past and follows first-year students traveling to Aomori, expanding the series' science-driven mysteries across two different parts of Japan.

> Light spoiler notice: This article discusses the premise provided in the official Volume 4 synopsis without revealing the resolution of its mysteries.

Quick Summary

• Volume title: Natsuzora ni, Michi o Sagashite.
• Full Japanese title: 夏空に、未知をさがして。 -デルタとガンマの理学部ノート４-
• Romanization: Natsuzora ni, Michi o Sagashite. - Delta to Gamma no Rigakubu Note 4
• Format: Light novel
• Series: Delta to Gamma no Rigakubu Note
• Volume: 4
• Author: Takuma Sakai
• Illustrator: Asagi Tohsaka
• Publisher: KADOKAWA
• Imprint: Dengeki Bunko
• Japanese release date: October 9, 2026
• Price: ¥968 including tax
• Pre-tax price: ¥880
• Book format: Bunko
• Pages: 264
• ISBN: 978-4-04-952183-2
• Preorders: Open through Japanese retailers
• Cover reveal: August 29, 2026
• Story season: Summer vacation
• New present-day location: Nagasaki
• Past-story location: Aomori / Tsugaru area
• Timeline: Present-day trip plus events from two years earlier
• Main group: Izuta and fellow Rigakubu students
• Core elements: Science / youth / everyday mystery
• Mystery approach: Scientific investigation
• Volume 4 themes: Travel, sea, mountains, and unknown mysteries
• Previous volume: Totteoki no Ronri o, Kimi to. - Volume 3
• Volume 3 release: January 9, 2026
• Series debut: November 2024
• Recognition: No. 3 in the New Bunko Division of Kono Light Novel ga Sugoi! 2026
• Manga adaptation: Ongoing
• Manga title: Yotte, Hatsukoi wa Shōmei Sareta.
• Manga artist: Lapp Tsutsumi
• Digital manga platform: KadoCom / ComicWalker

Volume 4 Cover and Announcement

Summer Vacation Takes the Rigakubu to Nagasaki

The official synopsis opens Volume 4 with summer vacation, travel, the sea, and mystery.

Izuta and his companions receive an opportunity to travel domestically and head to Nagasaki, where they begin investigating a small mystery hidden in the area.

As always, the group approaches what they find through observation, reasoning, and science rather than immediately accepting a more extraordinary explanation.

Another Story Reaches Two Years Back to Aomori

Nagasaki is only one side of the new book.

Volume 4 also turns the clock back two years, following students who were first-years at the time as they traveled outside the prefecture to Aomori. The promotional post specifically contrasts the sea of Nagasaki with the mountains of Tsugaru.

Exactly how these two trips connect has not been disclosed, leaving the relationship between the present and past mysteries for the novel itself.

The Mysteries Are Still Solved “Scientifically”

Since Volume 1, Delta to Gamma no Rigakubu Note has blended high-school life, science, romance, and everyday mysteries.

Rather than treating the unexplained as automatically supernatural, its students examine odd events through scientific knowledge and logical investigation.

Volume 4 preserves that identity while moving the fieldwork well beyond the familiar school and Tsunanagai setting.

The Series Is Growing Across Novels and Manga

Delta to Gamma no Rigakubu Note began through Dengeki Bunko in November 2024 with Yotte, Hatsukoi wa Shōmei Sareta.

It later ranked No. 3 in the New Bunko Division of Kono Light Novel ga Sugoi! 2026, followed by its second and third novels.

A manga adaptation of the first novel is also underway, with Lapp Tsutsumi handling the manga and Asagi Tohsaka credited for the original character designs. Its digital chapters are available through KadoCom.

Volume 4 Arrives October 9

KADOKAWA lists the fourth novel as a 264-page bunko priced at ¥968 including tax.

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Preorders are already open at multiple Japanese retailers, including KADOKAWA's own store.

The same month will also see the first collected volume of the Yotte, Hatsukoi wa Shōmei Sareta. manga adaptation, currently listed for late October.

Why This Matters

1) The mysteries move beyond the school grounds

Nagasaki and Aomori offer entirely different environments for the series' science-driven investigation format.

2) Two timelines create a wider mystery

A present-day journey and events from two years earlier leave room for cause-and-effect connections that may only become clear once both sides are revealed.

3) The franchise is expanding in two formats at once

As the original novels reach Volume 4, the manga adaptation of the first book continues to give new readers another entry point.

Light Novel Information

Additional confirmed information currently includes:

• Volume 4 is officially titled Natsuzora ni, Michi o Sagashite.
• the new cover was revealed through the official series account on August 29, 2026
• the Japanese release is scheduled for October 9, 2026
• Takuma Sakai remains the author
• Asagi Tohsaka provides the illustrations
• KADOKAWA publishes the book under Dengeki Bunko
• the official price is ¥968 including tax
• the book contains 264 pages
• its ISBN is 978-4-04-952183-2
• KADOKAWA Store lists dimensions of approximately 105 × 149 × 12.6 mm
• the story takes place during summer vacation
• Izuta and his group travel to Nagasaki
• they investigate a local mystery using a scientific approach
• another part of the book is set two years earlier in Aomori
• official promotion specifically mentions Nagasaki's sea and Tsugaru's mountains
• the exact connection between the two timelines has not been explained
• Volume 1 launched on November 8, 2024
• Volume 3 launched on January 9, 2026
• the series ranked No. 3 in the New Bunko Division of Kono Light Novel ga Sugoi! 2026
• the manga adaptation is drawn by Lapp Tsutsumi
• digital manga chapters became available through KadoCom in 2026
• Manga Volume 1 is currently listed for around October 27, 2026
• official Volume 4 information is available through KADOKAWA
• series information is available through the Dengeki Bunko Special Site
• the manga is available through KadoCom
• the cover reveal is available through Official X

Closing

Natsuzora ni, Michi o Sagashite. expands Delta to Gamma no Rigakubu Note into a summer travel mystery stretching from Nagasaki's coastline to the mountains of Tsugaru, while keeping science at the center of how its characters approach the unknown.

Volume 4 launches in Japan on October 9, 2026. The central mystery, the precise connection between its two timelines, and broader retailer-bonus details remain for later announcements and the novel itself.