Diamond is Unbreakable

Diamond Is Unbreakable (ダイヤモンドは砕けない) is the fourth part of JoJo's Bizarre Adventure, published in Weekly Shonen Jump between 1992 and 1996.

In 1999, the Arrow, manifesting latent Stand abilities, travels throughout Morioh, Japan; as high schooler Josuke Higashikata (illegitimate son of Joseph) and friends seek out the culprits of a series of homicides.

Plot Summary
An older Jotaro Kujo travels to coastal Morioh, Japan, in order to meet high school freshman Josuke Higashikata and link him with the inheritance of Joseph Joestar; his aging, absent father. Once Josuke displays his Stand, Crazy Diamond, Jotaro shares the news of a paranormal threat in Morioh; revealed to be the result of the arrival of the Bow and Arrow, an ancient artifact inciting Stand development in suitable individuals, as delivered by one of Dio's henchmen.

One of the Arrow's targets, the criminal Anjuro, uses his new Stand Aqua Necklace to exact revenge on Josuke's grandfather Ryohei, a police officer, for putting him in prison. Angelo succeeds in murdering Ryohei before he is defeated by Josuke; upon which Josuke pledges to defend Morioh against the potential threat posed by the Arrow's presence.

Josuke and Koichi Hirose find a dilapidated house when Koichi is struck by the Arrow, unleashing his Stand, Echoes. The culprits, Okuyasu and Keicho Nijimura, reveal that their father, formerly Dio's henchman, was disfigured and disabled by Dio's cells upon his death, and that Keicho aims to create Stand users in the hope that one of them will be able to kill him. Josuke manages to proves to them that their father is at least partially human, dissuading them from their activities.

However, this group is attacked by the Stand Red Hot Chili Pepper, which kills Keicho, and takes the Bow and Arrow. Thus, Okuyasu joins Josuke, Jotaro and Koichi in locating this second murderer. After Joseph Joestar arrives in Morioh, they are finally able to defeat user Akira Otoishi. The Bow and Arrow are then taken into Speedwagon's custody, fostering a temporary peace.

Koichi takes priority when he is kidnapped by psychopathic classmate Yukako Yamagishi, determined to make him love her. Escaping, his Stand evolves into a stronger form. Yukako, impressed, decides to pursue Koichi in a more consensual though no less determined manner.

Koichi meets manga author Rohan Kishibe, a target of the Arrow, and user of the Stand Heaven's Door. The pair later meet the ghost of a young girl named Reimi Sugimoto, who was murdered by a serial killer over a decade before. She begs Koichi and Rohan to find her killer, who has stayed in Morioh and is still preying on innocent people. This person turns out to be Yoshikage Kira, a serial killer who destroys women's bodies using his explosive Stand Killer Queen, keeping their hands as ghoulish romantic objects.

When Josuke's new friend Shigechi discovers Kira's secret and is killed, the allies of the story congregate and agree to collaborate in locating Kira and ending his depredations.

Jotaro and Koichi eventually find Kira, but before he is completely defeated he is able to force Aya Tsuji, user of the Stand Cinderella, to alter his features; killing her before she may inform the group as to his new appearance.

Shortly thereafter, the ghost of Kira's father deploys a second Stand-creating Bow and Arrow to create an army of Stand users to protect his hiding son, in the buildup to the confrontation between Josuke's allies and Yoshikaga Kira.

Trivia

 * Araki mentions that some of the enemy concepts in Part IV were leftovers he was unable to include in Part III.
 * According to Araki, the broad plot of Part IV was inspired by real murders occurring in Japan at the time.
 * This arc is notable for taking place both in Japan and in a fictional setting, without travelling to any real locations. The plot is mystery-oriented, not travel-oriented, as characters balance everyday life with encounters with new Stand users and the search for Kira.
 * Exhausting the Tarot and Egyptian mythology in Part III, Part IV primarily references popular (rock) music in its names.
 * For its inventiveness and artwork it occupies a high rank among fans.
 * Araki states that it is possible that JoJo's Bizarre Adventure could have ended with Part III, since the resurrection of Dio was the last plot element of precedent.
 * Sometimes when a character is defeated the character will "shrink" from being around Josuke's height as a villain to Koichi's height as an ally (i.e. Tamami Kobayashi, and Toshikazu Hazamada).
 * Among the community commonly accessing scanlations, Part IV is infamous for the "Duwang translations"; the only scanlations available until recently. A project by two Chinese students who were assigned to translate something into English, they failed their assignment due to how poor the translation is. Its many errors include original names for characters (e.g. Shigekiyo Yangu as "Fatty", and the setting Morioh as "Duwang") and lines like "What a beautiful Duwang" and "Get a feeling so complicated".
 * The title of this arc was formerly translated as Diamond Is Not Crash.