I have a 4:3 Monitor, and I desire to play the game in fullscreen mode with 16:9 letterbox mode turned on. but when i run it in this mode the letterbox isn't turned on and instead the game is fullscreen and squashed vertically. 16:10 letterbox works, but 16:9 does not.

is there a way to fix this?

After you choose 16:9 mode in your game...

...go to your GPU control panel and enable GPU scaling and click maintain aspect ratio. (Nvidia way)

It should be similar to AMD GPUs.