Mini 4 is going on 3 years, not 4. Mini 3 had A7 and Mini 4 has A8 and double the RAM (2GB instead of 1GB). Also, Mini 4 has a laminated display, so the design is thinner than the Mini 3. Mini 2 & 3 will likely cease to be supported on iOS when iOS 13 debuts next year. Mini 4 will live to see another update, but its future beyond that remains a mystery (it’ll likely not be supported on iOS 14, but who knows).Desperate wanting a new iPad Mini! It been going on 4 years or a long time since Apple updated it with a new SoC. The Mini 3 & 4 used the A8. Apple added more memory to the Mini 4. Only difference.
If Apple has any intention to keep the Mini around longer, they’ll need to refresh it this fall or in the spring of 2019. Otherwise, it’ll likely be 9.7”, 11” and 12.9/13” going forward.