what is the model number for ipad mini 4

Where to find it was mentioned. There's the case number and the official model number from settings. The latter will vary depending on color, capacity, and intended distribution country for functionally identical devices.

My gold iPad Mini 4 128 GB Wi-Fi is MK9W2LL/A. The first M means it was a new retail device. K9W2 is the gold 128 GB Wi-Fi iPad Mini 4. LL means distribution in the US or Canada. /A means that it's the first revision.

Settings > General > About > Model

Try https://www.lifewire.com/find-your-ipads-model-number-1994687

iPad Mini 4

For Wifi models A1538

For Wifi AND cellular data models A1550