Mac mini (Late 2014) and 4K TVs
Did you try the final solution on that thread? 

"Final update. Got my new 2014 Mac Mini now, works w/o problem with my 4k Hisense TV now.

Maybe, worth a note ...

Initially, boots in 4k but then switches to 1080p after the OS is loaded.
To get 4k at the OS level, one has to do this:
1. Switch to 24Hz, 25Hz or 30Hz under Scaling
2. Now, the 2160p resolution emerges in the list (3840x2160)
3. Select it
4. Note how 50Hz and 60Hz now disappeared from the list

It is straightforward to do. But maybe not as intuitive to everybody as Apple may think."