Very simple. If you have an iPad mini4, it lacks the specific screen hardware with which the Pencil interacts with the iPad.
Put another way, your iPad mini4 does not support Apple Pencil.
Here is the definitive compatibility list for Apple Pencil and iPad:
Apple Pencil (2nd generation)
You can use Apple Pencil (2nd generation) with these iPad models:
- iPad Air (4th generation)
- iPad Pro 12.9-inch (3rd generation) and later
- iPad Pro 11-inch (1st generation) and later
Apple Pencil (1st generation)
You can use Apple Pencil (1st generation) with these iPad models:
- iPad (8th generation)
- iPad mini (5th generation)
- iPad (7th generation)
- iPad (6th generation)
- iPad Air (3rd generation)
- iPad Pro 12.9-inch (1st or 2nd generation)
- iPad Pro 10.5-inch
- iPad Pro 9.7-inch
Not sure which iPad you have? Identify your iPad model.