I have a Raspberry Pi 4 with 8 GiB of RAM, connected to a 4K monitor (Dell U3818DW). Enabling 60 Hz refresh rate should be as easy as adding the hdmi_enable_4kp60=1 flag to /boot/firmware/config.txt.

But there seems to be a problem parsing the EDID information, because the resulting mode only affords 30 Hz.

Fortunately, we can program the mode ourselves:

[pi4]
# Enable 4K@60Hz, single monitor
#force_turbo=1
disable_fw_kms_setup=1
max_framebuffers=1
hdmi_enable_4kp60=1
hdmi_cvt=3840 1600 60 7
core_freq=600
core_freq_min=600

Note that core_freq has to be set to 600 in order to provide the necessary bandwidth for the mode. This is usually taken care of by setting hdmi_enable_4kp6h=1, but not in this case.