New ipad mini 4 unusable because it's no longer supported?

The iPad mini4 is currently classified in Apple's parlance as "vintage" - but not yet obsolete.

The most recent version of iPadOS supported by the iPad mini4 is iPadOS 15.8.3. The mini4 does not support iPadOS 16 or later.

While many mainstream App Developers have ceased to support iPadOS 15.x or earlier - and have withdrawn older versions of their Apps - the App Store App itself should still be usable for downloading Apps that remain available for your iPad mini4.

Whilst many App titles remain compatible with your iPad, if you want or need to access to Apps requiring the most recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model.

To reiterate, your iPad is incapable of update to iPadOS 16 or later.

Rolling-back to an earlier version of iOS/iPadOS is also not possible.

All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed.

Consider that iOS/iPadOS updates include both functional and essential security updates. Restoring an earlier version of the OS would undermine the security of your device and your personal data.