For completionists, Yakuza 4 offers a comprehensive Completion List that tracks your progress across various categories. This includes completing all sub-stories, mastering mini-games, eating every dish at every restaurant, and collecting various items. Achieving 100% completion is a lengthy but rewarding endeavor.
Key collectibles include: Locker Keys (found scattered throughout Kamurocho, often sparkling on the ground, which unlock items from the lockers near the Millennium Tower), Weapon Blueprints (found through sub-stories or specific challenges, allowing you to craft powerful weapons), and various trophies/achievements tied to specific in-game feats. Regularly check your Completion List in the menu to see what you still need to accomplish. Some items are only available during specific chapters, so keep an eye out!