I've just installed on a Pi4.

The Pi correctly identifies the monitor as 4k:

kali@kali:~$ /opt/vc/bin/tvservice -s
state 0xa [HDMI CEA (95) RGB lim 16:9], 3840x2160 @ 30.00Hz, progressive

However, the desktop appears in 1080p and doesn't offer any other resolutions.

Any hints on where to look to get the desktop in 4k?