What type of Memory does M4 Mac mini support

Apple M4 uses LPDDR5X DRAM.

Some LPDDR5X configurations can have on-chip ECC, Link ECC, or both, and Apple has at least one error-correction-related patent in this area.

Whether Apple has implemented either or both on-chip or Link ECC with M4 is not known. I’d tend to assume not.

ECC memory is used on Apple Mac Pro Intel Xeon models, and on Apple Xserve Intel Xeon models.

Unlikely. Apple only ever used ECC memory on a few high end Macs. And since Apple Silcon unified memory is part of the SoC it is very unlikely that it is ECC. If it had ECC, Apple would have said so in the specs.