How can I force letterboxing on my display?
For anyone googling this in the future and wanting an answer...
If you have Nvidia Control Panel, go to Display > Adjust desktop size and position
Go to the size tab, select a custom resolution that's native to your display (in either height or width) but with one side adjusted to 'fit' the target ratio.
In the case of 1920x1080 to 4:3 this would be 1440x1080.
Then, in the Scaling tab select the following:
"Select a scaling mode:" - Aspect Ratio
"Perform scaling on:" - GPU
Check off the override option.
Now if you load up a 4:3 game and go full screen, you'll maintain the black bars on the sides.
Cheers.