They are literally the same dimensions, but the rear mic is in a different location (centered on the back instead of near the camera), so the Mini 4 case would cover that mic. I doubt that would have much of an effect though... and you can still find the Mini 4 silicone back case here: https://www.amazon.com/dp/B01576NMUG/ref=twister_B017B37BNW?_encoding=UTF8&th=1
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They are literally the same dimensions, but the rear mic is in a different location (centered on the back instead of near the camera), so the Mini 4 case would cover that mic. I doubt that would have much of an effect though... and you can still find the Mini 4 silicone back case here: https://www.amazon.com/dp/B01576NMUG/ref=twister_B017B37BNW?_encoding=UTF8&th=1