Windows can not manage to have different DPI 1080p and 4K screen. However, it is impossible not to have a magnification of 1080p but 25-35% on Dell 4K (P2715Q) intimidate. Windows 8.1 increases on both screens. I have clicked out that it will not change on both screens, and only on 4K Dell P2715Q.
I have Nvidia GTX 680 and latest drivers 347.88! Windows is 8.1 Pro with Mediacenter.