It doesn't say that an iPad Air 4 uses the 1st gen. Maybe you read that the 1st gen is used with the iPad Air 3
Apple Pencil compatibility - Apple Support
The below below which is from the apple website, clearly explains the which pencil is compatible with which 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