As of Patch 13.01, the method to get 4:3 true stretched res (often called the TenZ method) is not working. We are not really sure why at the moment, or if this was an intentional change as it wasn't in the patch notes.
Important: if you previously used the TenZ method you'll need to UNDO those steps (put your GameUserSettings back to default).
You can use any resolution that is a 1.45:1 ratio. Common ones are:
1568x1080
1280x882
1124x768
It's about halfway between native res and 4:3 res.
If you have any questions, drop those down in the comments below and I'll answer as many as I can.