A user reported in our forums:
https://forums.puri.sm/t/no-4k-output-with-external-monitor/6653

I tried that some time ago (back in May IIRC) and it worked on the 4k TV we have in the office. I tried that with my L13 laptop and my home built kernel and Todd tried with his L13 and vanilla PureOS and both presented 4k@30Hz options. Current PureOS with PureOS kernel and upgrade 5.3 kernel does not come present a 4k option in Gnome. I checked with edid-decode, the screen is properly detected and 4k resolutions are defined in EDID.

I tried on some laptops here locally, I am pretty sure this is unrelated to hardware. I can not exactly pinpoint the time of breakage though. Interesting the latest PureOS booted from USB does work (IIRC this is the latest ISO).