Multiple places online I have found the following:
iPhone 6S: iPhone8,1
iPhone 6S Plus: iPhone8,2
iPad Pro: iPad6,8
But I cannot find any reference to the iPad Mini 4. Does anyone know where Apple keeps that documentation?
Having hard time finding iPad Mini 4 hardware identifier
Apple doesn't document these identifiers. In general, there shouldn't be any need to use them. If you're testing because you require some particular feature of a given model, you should be testing for that capability not for the model identifier.
Perhaps if you mentioned why you need them, someone could point you to a better way to accomplish what you want to do.
It is just how our platform identifies the model for display in our reporting. I will wait until they are on Wikipedia: https://en.wikipedia.org/wiki/List_of_iOS_devices