If you have a Dell 4k display and a 900 series graphics card, and encounter screen flickering, check and see if you have a program called Dell Display Manager running. If so, under the Basic tab, change the screen characteristics from "Auto" to "Manual", then set the mode to "Standard". This should clear up any flickering issues.

You can find more details about the Dell Display Manager here: https://downloads.dell.com/Manuals/all-products/esuprt_electronics/esuprt_Display/dell-s2340l_User%27s%20Guide2_en-us.pdf