DIAGNOSIS That's the infamous 'IPS glow' or backlight fluctuation turning into an actual fault, often related to the monitor's Adaptive Sync or response time settings interacting poorly with the signal.
FIX First, disable any variable refresh rate. In the OSD, go to Game Adjust -> FreeSync and turn it OFF. Also, in Picture Adjust -> Response Time, set it to 'Fast', not 'Faster'. Then, on your computer, ensure your output (from GPU settings) is set to 60Hz, not 59.94Hz, and that the color format is set to RGB/Full Range, not YCbCr.
IF THAT DIDN'T WORK The issue could be a failing DP or HDMI cable incapable of handling the 4K bandwidth. Swap in a certified Premium High Speed HDMI cable or a VESA-certified DisplayPort 1.4 cable. The ones that come in the box aren't always the best.
HEADS UP If the flickering is a rapid, localized shimmer, you might be seeing the physical anti-glare coating defect some units have. That's not fixable via settings and would require a panel replacement under warranty.