may I install iOS10 on iPad mini 4

You wouldn't want to do this, anyhow.

As of the beginning of 2019 and continuing into 2020, many, MANY third party app developers are all more quickly and completely permanently dropping older app support for all old 7, 8 and 9-year iPads that are running 2-3 year old and earlier iOS versions ( iOS 10 and earlier ).

Third party app developers are removing all of their older, legacy versions of their apps from Apple's App Store servers, now.

Third party app developers no longer want to support these 8 and 9-year old or older iOS devices, any longer!

Older versions of apps are, soon, going to be gone for good, now!

So finding older versions apps for older iPads, while still possible, currently, will be a very tough challenge, now, in 2020.

Older versions of apps will be completely gone, FOR GOOD, at some point, in the very near, foreseeable future.

You can’t. IDevices can only install the most current supported operating system and for a mini 4 that would be iPad OS13