A:AnswerIt might be the hdmi cable. Some aren't made for 4k. The ps4 also needs to be set to 4k if it hasn't been yet. Maybe the upscale setting on the ps4 is turned on? That should be turned off. Hope this helped.
A:AnswerNo, the maximum refresh rate at certain resolutions is 75Hz, with a majority of resolutions maxing at 60Hz. Please refer to the supported timing list in the manual linked here. https://dlcdnets.asus.com/pub/ASUS/LCD%20Monitors/VP28UQGL/ASUS_VP28UQG_QGL_English.pdf