Grand Blue Season 4 anime was officially announced immediately after the third season's finale finished airing in Japan.
Images
Frantic Anime Brief
Frantic Anime is tracking “Grand Blue Season 4 Announced With Teaser Visual”. This update is filed under New Seasons & Sequels, Episode & Cour News, Announcements. Related developments will stay grouped on Frantic Anime so readers can follow the story without leaving the newsroom.
Reporting provenance: Anime Corner.