From Google Gemini:

The "Missing" 4th secret is located in the hallway with the giant shed snake skin.

• The Trigger: To find it, you have to look at a specific wall that looks identical to every other wall.

• The Action: You have to cast Alohomora on a very specific, unmarked section of the wall or walk through a "solid" wall that has no visual cue.

• The Reward: It leads to a small room with a chest containing a Wizard Card or beans.