It is a 1/2 inch light sensor for the main camera.
The Fairphone 4 rear cameras are the real deal. A replaceable pair of premium lenses for all the details (48MP main) or the bigger picture (48MP ultrawide). Add a light and color sensor for blazing fast autofocus and top it off with OIS for a shake-free shooting experience
Fairphone 4 Rear Cameras - Spare Part | Fairphone