It depends on what you want. By keeping the same vertical FOV you can essentially use the same sensitivity, as the 4:3 view is exactly the same as the 4:3 portion of 16:9.
Or you can keep the same horizontal FOV (if possible) and match the sensitivity based on what you see. So for instance flicking to the edge of 16:9 is the same as flicking to the edge of 4:3.