There’s a Kickstarter for a Mac Studio SSD upgrade but it still requires a bit of work. It’s unclear if these modules will work with the M4 Mac mini.

The Satechi Mac Mini Hub upgrades storage for a fraction of the Apple cost. The Mac Mini Hub increases Mini's usability and ...

Popular Mac repair technician dosdude1 has performed the first SSD storage upgrade on the new Mac mini, showcasing that it is indeed possible.