Which honestly seems pretty fair. 4K wasn't mainstream in 2013, you needed to spend a lot of money to buy a 4K display or projector. [0]
I wouldn't say that most people purchasing a Mac Mini are planning to use a 4K display with it, considering its target market is people who want to buy a Mac (without an integrated display) but don't want to shell out for a Mac Pro.
The Mac Pro not supporting 4K is a problem, one that has been brought up by many other people as an example of how Apple is ignoring the Pro market segment.
[0] http://www.alphr.com/news/382774/asus-takes-pre-orders-on-fi...
4K display prices are down, way down, and well within the normal consumer range now - you can pick up a very nice 4K monitor for $300 or less. It's not unreasonable to expect a Mac being sold new in 2017 to be able to drive it...
I have a late 2013 MBPR (NVIDIA GT750M) with an external 4K monitor in portrait mode connected via DisplayPort, plus the laptop's display. Both are running at 60Hz. I don't see any sluggishness or mouse lag at all; everything is very smooth.
I mostly run Windows 10 on this machine, but it also works fine when I boot in to OSX.
A while back I tested a 2014 MBPR with the AMD GPU vs. my late 2013 model with NVIDIA, and the AMD was a bit faster.