You should set GUI to 1080P @ 60Hz or 50Hz. I use 50Hz, as I use IPTV which is 25FPS, so there’s no need for refresh rate changes every time I play\stop a channel.
When you play 4K content, the refresh rate should match the content.
24Hz for 24FPS content, 50Hz for 25/50FPS and 60Hz for 30/60FPS.