Hello,
I have a Raspberry Pi 4 (rev 1.2), a Philips 436M6VBP monitor and an HDMI cable that I think should support 4k@60Hz (I can't find the reference anymore. But the eshop where I bought it with my rasp only has mHDMI cables compatible with 4k@60Hz, so I guess mine is also compatible). When I add hdmi_enable_4kp60=1 option, the monitor doesn't detect any video input. Without hdmi_enable_4kp60, LibreELEC run fine at 4k@30Hz. Btw I don't see any monitor setting like "HDMI ultra HD" or alike (like said in No 4K resolution on RPI4 )