Before we get ahead of ourselves, it's worth pointing out that the Mac Studio (M2 Ultra) has a similar storage setup, but custom upgrades are difficult to do: the computer uses Apple's own bespoke SSDs, you can't just slot in any off-the-shelf replacement.

Apple isn't soldering the SSD to the logic board in the M4 Mac mini, but given the company's history with Apple Silicon, don't expect upgrades.

On X, L0vertodream also posted on X a Mac mini teardown. L0vetodream states that “it is indeed a 1-slot dual-slot with a maximum particle size of 1TB. It uses a BGA315 hard disk and the maximum upgrade capacity can be up to 2T” (translated).