I suspect the reason few noticed that the quad-core Minis were discontinued is because you could only buy them at the very top end of the mini product line, and they were never available with discrete graphics, likely due to TDP issues.
In
2011, the quad-core mini was primarily available in the server version. (Even though having a quad-core processor is unlikely to help any of the tasks you're going to use a Mac OS X server for.) It had an i7-2635QM in it, which was a 45W chip, compared to the 35-watt chips shipping in the dual core Minis. Tray price, $378. This model of the mini cost $999.
In
2012, the quad-core mini was made available more readily in a non-server version. It used a i7-3615QM, which was a 45W chip with a tray price of $378.
The best quad-core chip for a presumed Haswell Mac mini would be something with HD5000 or better graphics, have a TDP of "about" 45W and would cost about $378. Such a chip just doesn't exist, and it would have been seen as a regression to only make the mini available at $1300.
For more information about the mac mini, you can take a look at
this thread I posted in last year. This stuff was all written before the Haswell-based Mac minis actually shipped, but it turns out, I was totally right. They even ended up using the exact chip model I mentioned, the 4558U.
The mini was never a particularly powerful computer, and the quad was always in a pretty weird place, because they never made it available with discrete graphics. Probably the best use case for it (the quad mini) would be compiling
really big projects remotely using the server component of xcode, if your main work machine was MacBook Air. However, the Mac Pro is really a better system for this task, and a good MacBook Pro or iMac would have blown even the quad-core mini out of the water, and would have not had the additional complication of setting up a compile server.
Actually, it's because Intel didn't have the right chips more than anything. The 4570R is way too expensive for a Mini.
At 65 watts TDP, a 4570R (which, yes, is also way too expensive, even for a Mini configuration that costs $1000) would absolutely
roast inside the Mac mini's slim frame.
Gigabyte has done it, and the reviews all say that it's insanely loud. (Also keep in mind that while you get a great quad-core CPU and some pretty good built-in graphics, your $500 (*the original price) doesn't get you RAM, storage, or an OS, and you get defects-only support.
I would have loved for Apple to take the mini closer to being a "desktop" but I suspect that they're getting a
great deal on the parts in the new mini and the 1.4GHz iMac, which is what prompted them to make the mini available for $499, down from its old $599 price.
Other i5-4570R and i7-4770R thoughts: what I'd love to see is a PC motherboard that has one of these chips soldered down and room for the customary 32 gigs of RAM. I'd buy the heck out of a variant on the DZ87KLT-75K in this configuration. Supposedly either with Broadwell or Skylake we're going to see some modular -R CPUs, that'll be interesting to see.
As we are all getting older (40)... as my dad used to fiddle with his ham radio when I was a kid it may regain interest.... I'd like to start my own BBS service.. I wanted that back then... but couldn't do it.. but now I am in a different position... but then again here comes that "time" thing!!!! 
