The only way i can think of doing this properly, is setting the largest 4:3 resolution you can fit to your screen, and then forcing the game to fixed aspect ratio.
I do have a 1920x1200 display and an nVidia GTX Card, so for me it would be done like this:
Set the game resolution to 1600x1200, this will fit nicely in the horizontal. Then go to the nVidia Control Panel. There is an option about desktop size. (please note that i only know the correct option in german, so i'm paraphrasing).
Choose the monitor, if you have more than one. The tab "scaling" shows you three options: aspect ratio (default), full screen, and no scaling. Select no scaling. In the dropdown directly below this, select GPU.
In the lower part you can preview, by selecting the chosen game resolution from the dropdown. This will show you where you will have black bars.
See the attachment (in german).
I assume something similar can be done with AMD cards, but i cannot confirm. Also some monitors may have options regarding the aspect ratio.
Be aware that this will affect all applications running on that monitor. Essentially anything that cannot run in your native resolution will now not be scaled to fit.
