Terms like 2k and 4k are confusing. A 4k monitor has a resolution of 3820x2160, you then get monitors that are 2560x1440 which is sometimes called 2k but this isn't really a general term, they are more often referred to as 1440p, much like how 1920x1080 is referred to as 1080p.
Given your GPU I'd recommend going for a 1440p one (2560x1440) - A single GTX 970 will struggle to run 4k whereas it will drive 1440p fine. This will still give you a better image than you'll find on a 1080p monitor and will give you a lot more space on the screen for work. You also shouldn't need to do any sort of DPI scaling on a 1440p monitor which you would likely need to do on 4k to make the text a readable size.
If you are seeing the "Virtual 4k" on the Qnix monitor eBay listings, don't bother with it, it's a worthless gimmick that the sellers put in the eBay listing to make it seem like a special feature. All it is referring to is forcing the monitor to work at a non-native resolution. If anything the image quality will be worse than running at the native resolution yet it will stress your GPU out more!
Also, don't feel like you need to go for one of those cheap Korean ones nowadays, it used to be the case that they were the only affordable option at 2560x1440 resolution however nowadays you can pick up a much better monitor such as the Dell U2515H for around the same price.
