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.