To find the only blacksmith (Masamune) that can upgrade weapons to Level four, you first need access to the Forest Castle. This area can be revisited later. In the Forest Castle: Royal Chamber, go right to find the exterior broken bridge. Drop down through the hole in the bridge to land on the walkway below. Move left and interact with the wall to discover a hidden block. Pull it back to reveal a secret path to the blacksmith. This blacksmith can upgrade Level three weapons to Level four for 10,000 per upgrade. His store carries the following three weapons: Machine Sword, Machine Axe, and Machine Spear. These are all weapons wielded by machine lifeform enemies.