Are you editing 16:9 footage in a 4:3 sequence? Editing directly in a 16:9 sequence might be a better workflow, but if you’re happy the way it is then your best bet is to add a letterbox matte over the top of the clip and the resize and reframe that underlying clip appropriately. You can make a letterbox matte easily by cropping a couple of slugs to fit, or you can make one in Photoshop, or by endless other methods. You could also use a letterbox generator, loads available off the web including one on my own web page at https://web.me.com/andymees