Depends on what you use it for. IF for gaming, then Air 2 would be the way to go. I gave my iPad 4 to my wife and got the Air2. Not much difference in terms of streaming videos, checking email and web surfing. Battery life seems better on the 4.
Not likely. If anything, due to its super thin design the battery in the Air 2 maybe slightly worse. Don't worry, the Air 2 battery is still excellent though.
As for whether to upgrade or not, if you are still happy with the 4 and its performance then I don't see the need to upgrade unless the device is really feeling old to you or you really want the weight and size reductions. For day to day use and with most apps I doubt you'll notice a huge difference of any at all.
Go play with an Air 2 at a local store if possible and see for yourself.
I figured the battery was bigger to accommodate the power hungry A5X in the 3 and the A6X in the 4 + retina display. (Battery life for all iPads appear to be around the ~10 hour mark suggesting the A7/A8X are far more efficient despite working from a smaller battery)