basically 3 ways to treat 4:3 footage heading for 16:9 land
Edge Crop, Letterbox, and Squeeze
Edge Crop is sizing the 4:3 footage to fit the full width of the 16:9 image area, whilst cropping off (and losing forever) the top and/or bottom of the picture that lies outside the 16:9 image area.
Letterbox is sizing the 4:3 footage to fit the full height of the 16:9 image area, whilst filling the left and right of the 16:9 image area with black (often called a pillarbox)
Squeeze is stretching/distorting the image to fill both the width and height of the 16:9 image area.