I want to record a screen recording of ubuntu in a VM at 4k for people with new monitors however I only have a 1080p screen.
Is there any way to turn the resolution of the VM in virtualbox up to 4k while still fitting it on my 1080p screen?
I have tried manually setting the resolution with xrandr but I get the error "Size index 3840 is too large, there are only 13 sizes" When I listed the sizes the highest one was 2560x1440 but trying to set this resolution failed too.
To be able to run a 4K virtual display in Virtual Box on a smaller screen we need to define the desired resolution (4096 x 2160) first (also see my answer there):
Calculate the modeline for xrandr
gtf 3840 2160 60
Modeline "3840x2160_60.00" 712.34 3840 4152 4576 5312 2160 2161 2164 2235 -HSync +Vsync
Define a new screen geometry mode:
xrandr --newmode "3840x2160_60.00" 712.34 3840 4152 4576 5312 2160 2161 2164 2235 -HSync +Vsync
Attach this mode to the Virtual Box virtual screen (e.g. VGA-0):
xrandr --addmode "VGA-0" 3840x2160_60.00
Apply this mode to the Virtual Screen:
xrandr --output VGA-0 --mode "3840x2160_60.00"
We can alternatively choose this new geometry from System settings... > Displays.
Note: Virtual Box should run in windowed mode on a screen having a lower resolution than the new geometry we had defined.
Sadly due to a bug in Virtual Box we may need to disable 3D acceleration in the machine's settings to be able to record the desktop with our favorite recording software.
Below is a scaled down example GIF from a 4K virtual desktop:
