Whenever I try and run a game in a 4:3 aspect ratio in full screen it defaults to my laptop's native resolution

Would I be able to change a setting in a tab, or apply a script?
Here’s an example of the issue

I’m assuming “Default is Native Resolution” is checked for you, but these options in Build Settings > Player Settings > Resolutions , are most likely the culprit.

If you want to change this with code at runtime rather than settings I suggest using Screen.SetResolution() in an awake or start method.