I wish they would just call these the 5th Generation iPad Pros.
1st Gen were the A9X based ones 12.9" and 9.7"
2nd Gen were the A10X based ones 12.9" and 10.5" and introduced ProMotion
3rd Gen were the A12X based ones in the current sizes with USB-C
4th Gen were the A12Z based ones in the current sizes with the ultrawide camera