Ninja Gaiden 4’s protagonist Yakumo has a singular slot for accessories at the start of his adventures. The game itself does not explicitly let on that the number of slots will be expandable over the course of the story.
This guide for Ninja Gaiden 4 is an explainer on how to acquire more slots for accessories.
Upgrade Accessory Slots
Only in Chapter 4 will you first be offered the option to increase accessory slots. Interact with the merchant Tyran, who up until now only had combat skills on offer under the Learn Combat Skills menu.
Mixed in with the rest is the new Art of Adornment Lv. 1 Passive ability, which will increase the number of accessory slots to 2, in exchange for 8000 NinjaCoin.
Eventually, more of these Arts of Adornment will become available.
Art of Adornment Lv. 4 can be found in a chest in Chapter 15: Fanged Gate. Just before you slide under the large gate, deviate to the right instead through a tunnel. Back on solid ground should have you right in sight of the chest holding the item.
That is everything there is to know about expanding the number of accessory slots is Ninja Gaiden 4.
