Along with the dramatic external design, the new M4 Mac mini also features multiple internal changes. According to early teardowns and user reports, the new Mac mini features a removable SSD. This doesn’t mean, however, that you can buy a new Mac mini and upgrade the storage after the fact.
On the iFixit forums, user KianWee Lim posted a picture of the M4 Mac mini’s internal design showing that the SSD is not soldered to the logic board. This suggests that the SSD is modular and relatively easy for a user to remove, similar to the Mac Studio.
Källa: M4 Mac mini offers modular SSD storage, faster base model speeds – 9to5Mac