They’re closely linked as whilst the Mini 4 is newer, they use the same processor family.
The Air 2 has a triple core A8X at 1.5ghz whereas the mini 4 has the Dual core A8 at 1.5Ghz. Therefore single core performance will be similar but multicore the air has the upper hand with an extra core available. Whether this is noticeable would depend on the apps you use.
Both have 2GB of RAM.