2.3b, Yosemite, Mac Mini 2012, 4K - Yes!
 

Delighted to report that I finally have 4K running off my Mac Mini 2012 thanks to the 2.3b build in Yosemite. The adapter is a Club3D USB 3.0 to DisplayPort 4K Adapter.

I also have another no-name USB 3.0 HDMI Displaylink running simultaneously.

I now have four monitors in total, three at 1080p and one 4K under Yosemite. I can only get Yosemite to run 1080p through the Displayport on the Mac Mini to a 4k display (AOC U2868), although it will run 2560x1440 to a Thunderbolt or Cinema display. Unlike OS X, under Windows 8.1 (Bootcamp), the Mac Mini 2012 will run 4k natively from the Displayport, so I have two monitors at 4k and two at 1080p under Windows from the Mac Mini.

An excellent result!