I've searched for this topic, but could not find anything on it.

I've had access to features in some games that allow one to force a 16:9 aspect ratio while using a 4:3 screen resolution. An example for a use of this would be this:

Using a 16:9 widescreen monitor, but a display of 4:3 800x600 resolution, and forcing the aspect ratio to match 16:9.

I am not asking how to force a 16:9 screen resolution. I am already capable of using resolutions such 1280x720, 1366x768, 1600x900, etc.

I am asking if there is a way in skyrim to force a 16:9 aspect ratio while using a 4:3 screen resolution, such as 800x600, 1024x768, etc.

The advantages of doing this are for performance. If the gpu is displaying a res of, say, 800x600 or 1024x768 on a 16:9 widescreen monitor, then being able to force that 800x600 resolution to display in 16:9 aspect ratio would gain performance due to the low resolution, while maintaining a natural 16:9 aspect ratio without having the stretch effects that are associated with using 4:3 resolutions on widescreen monitors.

As I said, I am capable of using 16:9 resolution, but that is not what I am asking how to force.

Thank you

If you use a 4:3 resolution, you will not have a native 16:9 aspect ratio no matter what you do. There will be stretching, black bars, stuff cut off, or whatever.

I understand that under normal circumstances, there will be those types of negative effects. However, there is a feature in some games that forces the display to have an aspect ratio of 16:9 while still being under a 4:3 resolution. There are console games that used to make use of this effect, and I am sure there are pc games that have used it as well

Screen resolution directly affects the aspect ratio. Some games, such as Assassin's Creed forces 16:9 aspect ratio via black bars, I can only assume that you want this effect, and I can't help you. You might want to try windowed game and tick the "show all resolutions" checkbox

However, your belief that it will improve performance is semi-false. GPU will, for rendering purposes, render a 800x600 image. The stretching part doesn't really require extra effort on the GPU part not to the point that it's a feasible way of improving fps (I'd think it doesn't even improve for half an fps). It is true that stretching the image would arguably require some processing power, but certainly not much.