He wants to crop the output so as to have a 4:3 output from a 16:9 screen resolution. Obviously his display is 16:9 but he wants OBS to only output the centered 4:3 window. This is possible as i have done it myself, i kept the shader on mine so its still got the shaders built in TV bezel, but it IS a 4:3 crop from a 16:9 image, this video below would normally have a bezel on the sides as well.
This is how my Retroarch looks normally, with TV speakers as the bezel.
And this is how it looks with my cropped recording made with the OBS integration, its not quite right as there is some empty space at the bottom so i need to tweak it, but its a 4:3 cropped output from a 16:9 display.
Treasure Island Dizzy-01.mp4