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The Armorer's Cookbook 4 is one of the many Key Items found throughout Elden Ring, located in the cask room of Redmane Castle in Caelid.
The Armorer's Cookbook 4 is a Key Item Recipe Book that will unlock extra crafting recipes once you have obtained it.
This includes the following recipes:
You can find the Armorer's Cookbook 4 in Redmane Castle at the southeast corner of Caelid. It is located inside a room full of casks and barrels near the main gate, guarded by two Bladed Lions.