IMHO, I'd really suggest that going to 4K for more screen real estate is a bad idea unless you're going to a monitor that is at least 32". Unless you have exceptional eyesight (as in the kind of eyesight no one has), unscaled usage of a 4K monitor is really not something that works well in practice for any stretch of time, especially for text intensive activities like programming. I've tried out 3 different 4K monitors for this purpose and the best results for font legibility were at 1.5X or 2X scale. My suggestion would be that if you're not looking to run scaled, there are lots of awesome large IPS monitors that you can get at half the price of a 4K monitor.
I've tried all kinds of scaled resolutions on both my rMBP and a 5K iMac in store, and all were imperceptibly as sharp as 1.5x or 2x at least for short term use. So I guess it depends on your visual acuity and level of scaling selected as to whether scaling is for you or not.
I'm expecting to run my 4K 27" displays somewhere between 2560x1440 (1.5x) and 3200x1800 (1.2x). I'll try various scaling settings over time to see what works. Maybe 1.5x will be my long term choice... I guess I'll find out.