I've been contemplating the same refurbished idea. One thing I'm uncertain about is the operating temperature range.
The RPi4 spec is  0 - 50C 
while the M1 2020 Mac mini  50° to 95° F (10° to 35° C).
  and the 2014  Mac mini is also  50° to 95° F (10° to 35° C)

Here in Canada I've operated my Odroid-N2 down to at least -10C, but I wonder if a Mac mini would be up to cooler temperatures? The Mac mini certainly does have a more robust case which might extend it's temperature range beyond the specs a bit.

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