Had a 4, was satisfied but it was a bit slow. Since my 4 could do everything an Air 2 can do, I was going to wait, but a sale at Best Buy got me to bite. After selling my 4, the true out of pocket expense was pretty reasonable.
The Air 2 is a very good tablet. Very fast, very light, very portable. I find myself using it alot more than my 4 because it is easier to carry around, easier to use, and offers a better experience than my 4 ever had. The 4 used to be a bit of a burden due to its weight and it was a bit slow, so I'd often rather use another device instead.
If you are happy with what you have, then wait. But coming from anything besides an Air 1, you'll notice an upgrade for sure.