The MacBook Air 11-inch laptop with Intel 5000 graphics and MAC OS X Yosemite 10.10.3 does not support 4k x 2k graphics on Dell P2815Q display.
MacBook Air 11 systems with Intel 5000 graphics may experience several display issues when connected to Dell P2815Q displays. The following issues have been reported:
The P2815Q has a 4k x 2k optimal resolution of 3840 x 2160 on DP/mDP or High Definition Multimedia Interface (HDMI) cable connection. The MacBook Air 11 has a max external resolution of 2560 x 1600 using MAC OS X Yosemite 10.10.3.
The cause of the display issues above is an inability of the MacBook Air 11 to fully support the display capabilities of the displays. The P2815Q displays were tested using systems that fully support the P2815Q display capabilities. No issues were noted when connected to systems with 4k x 2k display support.
Troubleshooting Notes: