Make your sequence anamorphic 16:9 (which you’ve already done).
create a new sequence with the same settings except don’t make this new sequence anamorphic 16:9.
Now nest your 16:9 sequence into the new 4×3 sequence by dragging the 16:9 sequence from the bin into the 4×3 sequence. This should now letterbox the image.
You are outputting in 16:9. Do you mean you want to ouput a letterboxed 4×3 image? Once you letterbox the output, it will no longer be true 16:9 widescreen.