The first version of iOS11 sucked big time on the Mini 4. The interface animations were a miserable lag fest. Later versions of iOS11 improved the fluidity a lot.
I took the plunge on iOS12 and I'm very impressed. It runs extremely smooth considering the fact that the Mini 4 was released in 2015. Battery life might have worsened slightly, but it's just a feeling. My devices has clocked +300 cycles, so it's not really new anymore.
I'm very pleased that Apple for once has optimized the performance on iPads. The Mini 4 doesn't really feel like a device released back in 2015.
I am using the Mini 2 and ios 12 it makes useable for me again for most things. Does not like loading lots of photos in a webpage but will render them okay which ios 11 just would not do it for the A7! It is good overall for the A7! A8 should be much faster and the Mini 4 has additional RAM too!