You might also want to do a reset smc/pmu and zap pram as well. I don't know what model of Dell 4k monitor you have, but you might also want to adjust the black level on the monitor itself. under color you may need to calibrate the display and adjust the native Gamma, as the default one for PC and Mac are different. If you hold down the 'Option' key and click on Display, Detect Display should come up, and you should get a variety of options, such as default for display and scaled. On the Dell 4k monitor side of things, have you turned on deep color (HDR?) and DisplayPort 2.0 ? On the Mac side of things, go to system preferences, then displays, then Color/Display. Okay, amyhr, maybe I can help you with that.
