Kinda is. Graphics performance of the mini 4 is closer to iPhone 6/6+ than Air 2 (A8X) or 2017 iPad (A9). Also, the single core performance jump from A8 to A9 was quite significant. Microsoft Excel is noticeably slower on the Air 2 and mini 4.
That said, I do see A8 being supported a bit longer. Apart from the switch from 32-bit to 64-bit, I believe what Apple does is test performance of older devices on the new iOS. If it meets a certain minimum level of performance, then it gets an update. Of course, that minimum level of performance seems a lot lower compared to the standards of most MacRumors forum members (at least the prolific ones).
I don't think 4 years was quite standard prior to the A5. The A5 marked the start of iOS device longevity. Also, it's 5 years support - iOS 5, 6, 7, 8, 9.
The A6-based iPhone 5 and A6X-based iPad 4 actually aged better compared to A5 devices. The A7 is also doing quite well on the latest iOS version. There are some lags and stutters here and there but nowhere near as bad as A5 on iOS 8+. Honestly, the main reason I upgraded my Air was lack of storage. Seriously, unless all one does is web browsing, streaming and have less than 20 apps installed, 16GB is way too small.