Can't get 4K working with DP

I have a brand new Macbook Pro 15" Retina and a Philips 288P6 4k-monitor. For a while everything seemed to work. But now when I connect the display with DP cable, OSX says 1080p is my maximum resolution. With HDMI I get 3840 x 2160 but only at 30Hz which is not nice.

I've tried pretty much everything I've found with google: zapping SMC and NVRAM, deleting com.apple.windowserver files etc. Nothing works. What could be the problem?