I have a Macbook Pro connected to a 27" 4K display at comfortable desk with a comfy chair at home, so the bigger iPad Air that I have doesn't really add much. My kid use it when she is allowed to watch a movie from time to time.
The Mini 4 on the other hand is a different animal. It's small and light enough to feel more like a hand held device for book reading, emailing, browsing, etc. when I feel like doing it while sitting in bed or on the sofa.
I was going to upgrade from the Air 1 to the Air 2, and sell the Air 1, then I realized that I don't use the Air 1 that much, and perhaps the Mini 4 would be more suitable for me. That turned out to be the correct assumption.