Life span of iPad mini 4

Impossible to say. Apple does not disclose update life-spans of devices.

Normally its around 7 years or so of iOS updates from year of release, so its likely iOS 15 is the last major iOS update it will get, but it's impossible to know for sure. It may get iOS 16 in the fall. There's no way to know for sure at this point.

If you want yes. You can try to get a refurbished Mini. Or a brand new iPad 9th generation, they are about the same price. The 9th gen was released in 2021. Unless you specifically want the smaller sized iPad mini.

10.2-inch iPad Wi‑Fi 64GB - Space Gray - Apple