How can I use the Raspberry Pi’s 60FPS setting with 4K (4k60)?

Enabling 4k60 means that it will be taken as standard, if available. We have noticed that this is not particularly stable at this time, and is not recommended. For most applications, this is not necessary.

In particular, the mechanism that is used to ensure that VNC can be used without a display attached (forcing HDMI output on HDMI1) seems to cause problems with this setting.