will an ipad mini 2 sim card work in an unlocked mini 4
Will a sim card from a ipad mini 2 fit a mini 4
Technically yes.
If both iPads are cellular models, then they both support nano SIM cards. Your cellular provider may (stress may) lock your SIM to a specific device, or if they supplied the iPad, may lock the device to their network. If the iPad has been purchased from Apple, the the device will be unlocked to work on any supported network.