I have two monitors on my desk, One of them has a very high resolution, but with noticeably higher latency than the other.
When using the high resolution / high-latency display, I notice that I start to feel more clumsy, frustrated and stressed, so (despite all of the gorgeous pixels), I tend to use it just as a 'side display' for static documents and reference material ...
I think latency could be a critical factor here.
On the other hand, take a look at your developer's postures: are they leaning/squinting into the screen? Are they using 1:1 pixels, or scaling?
What does the retina get you when one is a developer?
Coming back to what I said, I have large monitors so that I can make use of and see the pixels. Any thing smaller would probably be a waste of 4K unless I had really good eye-sight.
But if serious, there may be some problems with adapting specific software to run HiDPI.