certainly possible, depends how you want the final image to look like as to how it's done though. if you wish to add black bands and preserve the original content completely, check the -vf expand= option in the documentation, this will allow you to increase the size of the bounding box of the image, up to a 4:3 ratio. one thing to note here is that (from what i'm aware of) image sizes should always be in multiples of 16 bits, so exactly scaling to a 4:3 ratio may break that, but a little distortion when strictly playing back to 4:3 isn't going to hurt (e.g. 4:3.12 is as good as dammit yeah?)
if you're looking to crop the sides of the image, then it's a -vf crop= operation, and you could also do a combination of both, so crop half the width down, and then expand the remaining image up to a 4:£ image, from say the resulting 14:9 image.
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