My honest guess is 0.0001%.
The Haswell architecture supports DDR3 memory, not DDR4. It would basically be a change of strategy if Apple were to introduce a new major architecture on the Mac mini, a product that does not sell in large quantities.
Historically, Apple has used their best-selling lines to introduce new technology. For several years, that would be the Mac notebook lines (and the iPhone for the handheld device space).
Apple has historically made the Mac mini as a headless MacBook, using power-sipping mobile processors. This is not the product line that would see a new RAM interface first.
The likely place for a new memory architecture introduction would be the MacBook or iMac lines.