Based on your reply it looks like you are in search of display scaling for the LUbuntu guest but keeping the 4K Windows 10 display setting.
I don't have Lubuntu, but try this from terminal. It will scale the display twice as big.
xrandr --output Virtual1 --scale 0.5x0.5
You may have to change Virtual1 to some other name depending on your setup. You can see the display name by entering xrandr without any parameters.
To go back to original scaling
xrandr --output Virtual1 --scale 1x1
You can also create custom screen resolutions (using xrandr with some input with cvt) that are not available although these would not be permanent (i.e. does not survive between logoffs/reboots).
For example you can experiment a 3:2 aspect ratio 1800x1200 with scaling of 0.75x0.75 with the following entered at Terminal
cvt 1800 1200 60
# 1800x1200 59.89 Hz (CVT) hsync: 74.57 kHz; pclk: 180.75 MHz
Modeline "1800x1200_60.00" 180.75 1800 1920 2112 2424 1200 1203 1213 1245 -hsync +vsync
xrandr --newmode "1800x1200" 180.75 1800 1920 2112 2424 1200 1203 1213 1245 -hsync +vsync
xrandr --addmode Virtual1 "1800x1200"
xrandr --size 1800x1200
xrandr --output Virtual1 --scale 0.75x0.75
Once you are satisfied you can set the xrandr as some script.