Improving guest resolution running on a Librem15


I have a Librem 15 v4 running PureOS and and I’ve been trying to set the guests resolution in kvm/qemu to the same of the host, i.e., 3840 x 2160 and --scale 0.85x0.85 through xrandr. I’ve been using spice and QXL, since that was the best graphics perfomance I achieved running videos. The problem is, whenever I try to set the display to 3840x2160 or scale to 0.85, I’ve got an error that seems coming from the X server. I’ve tried to fix that editting a xorg.conf file in /etc/X11/xorg.conf.d of the guest and upgrading vgamem to 65536 but with no success. Is it really possible setting this up? I’m just an average linux user, so I don’t master thoroughly these settings. The best resolution that I’ve got in the guest so far has been 1920x1080. Is this the limit for a guest?