The "quest" For Kojiros fourth weapon is in Story mode, on his fourth stage. In order to unlock his weapon, you must clear 300+ kills, as well as kill all Vagabonds (including any that spawn), help the Ieyasu retreat, kill Hideyori, and reach the southeastern gate within ten minutes. To do this you will need a fast horse and a decently leveled Kojiro. At the start of the battle, clear all the Vagabonds around Ieyasu. Then, run back and clear the escape point. Wait for two more Vagabonds to appear. Kill one, which will cause at least six more to appear. Kill them and wait for Hideyori to spawn in the southeastern maze-like area. Find and kill him, then quickly go to the southeastern gate. Musashi will appear in a short intermission sequence. A notice will then appear, showing you the location of your new weapon. Make sure to get your new weapon before killing Musashi.