Do you understand how cores work? Honestly, I feel like people here have no idea how processors function. More than likely, you will never be using four cores at once.
Spoken like someone who has never used a cluster of Mac Mini machines for a render farm.
If you are really in a situation that calls for tons of cores, buy a computer designed for high-level usage like an iMac or Mac Pro.
Spoken like someone who has never tried to rack mount a stack of iMacs or Mac Pros.
Not everybody uses Mac Mini machines as desktop computers. They're quite popular in compute clusters and server farms because of their form factor. And in those environments, the extra cores matter. A lot.
Of course, if your compute farm also depends on GPU power, you might still be better off with the newest Mini, but if you're CPU-bound, the previous model is a much better deal. Unfortunately, I'm CPU-bound AND GPU-bound (first one, then the other), so I'm pretty much equally screwed either way.