Your original comp is 16×9. Wide.
The client wants 4×3, which is 16×12. Narrow.
You made a mistake by working in 16×9, or your client changed the specs late in the project.
You have two choices if you want no black bars:
1) squeeze the original so it all fits on the screen. This will look strange.
2) cut off the left and right sides of the original. This will lose image on the sides.
Here is one way to render:
1) drag the comp into a new DV NTSC comp.
2) scale so the original comp fills the new comp (cmd+opt+f), or scale so the original comp is cut off on the sides (cmd+opt+shift+G).
If you can convince the client to use black bars, then scale the original comp (in the new comp) using cmd+opt+shift+H.
Render the DV NTSC comp.