Mini 4 vs. Air 2?
Really comes down to size... Portability vs. larger display. A big factor there is how much browsing you do-- and how sensitive you are to small text. I really enjoy the Mini-- typing this on a 2 right now--but many web sites have text too small to read comfortably. Yes, you can pinch-zoom or rotate the screen, but often that leaves the page and text off the screen and you end up having to scroll back and forth! Not much fun! Apple's own support forums are one of the absolute worst in this regard.
If you plan on watching videos and streaming sports, then you may well find the Air is a more compelling buy, especially if you are likely to share the view with others or, put it on the table in front of you -- same for shared games, too.
But the Air cannot match the intimacy, curl up on the couch feel of the Mini.
And, if you're into photography at all, in our experience, the Mini is hands down the winner--easier to hold, quicker to grab and reposition, and the right size for displaying the photos themselves.
There really is only one solution to this, you know, and that's to buy both!