Even although the Mini 4 is just out, it does feel like last years tech. What seems apparent is that Apple is pushing the iPad refresh cycle into a 18 months to 2 year cycle. I was surprised at the time when Apple released the Mini 2 and the Air at the same time with the same processor (A7), albeit, the Mini 2 was clocked slightly lower. They have obviously decided that they want to create 3 distinguished layers of iPad experience. The Mini, the Air and the Pro. The Pro effectively got the chip that should have normally went into the Air 3 this year, hence the reasoning why an Air 3 wasn't released.
It now looks like the Mini 4 won't be refreshed until 2017, while the Air may get a new lick of paint early next year, but agree with others stating that iPad's are being movied onto a near 2 year cycle.
With regards to the Mini 4, it's a good machine, but is it worth the extra money up from the Mini 2, or what you can pick up the Mini 3 for? I don't think so. You could probably get an Air 2 for the same price as a Mini 4.
And if we are looking at a longer refresh of models (ie every 18-24 months), then we are likely going to see huge processor changes after 2 years than only 1, but what seems likely is that the iPhone will lead the charge with the better processor, while the iPad's will be getting the previous years tech.
So in Spring 2017, we are more likely going to see a Mini 5 with an A9, while the Air will have the A9X and the Pro will get the new A10X.