To output 4K to a DisplayPort display at 30Hz, your monitor must support the resolution and refresh rate. If your monitor also supports 4K at 60Hz (DP 1.2 or higher), you may also need to set the monitor to work at 30Hz.
To resolve this issue, use the buttons on your monitor to change the DisplayPort settings to DP1.1. For more information about how to change your monitor settings, refer to the website of the manufacturer of your monitor.