You can animate the position of the crop box to pan and scan a 16:9 clip to a 4:3 format. Pan and scan is a technique widely used for resizing 16:9 clips to a 4:3 resolution when the area of interest in the shots is to one side of the 16:9 frame. You can follow the area of interest in the shot by animating the crop box along the X-axis.
To pan and scan a 16:9 clip to a 4:3 clip: