ok, just FYI, 4:3 IS the size of the TV so there would be no letterbox.
in order to make 1.85:1 to 4:3 you have to "zoom in" on the 1.85:1
cutting off the picture on the right and left sides.
So on that program you are using, if there is a ZOOM feature use that.
other wise you'll have to re-render the movie in FCP or iMovie or something.
personally, i think you should leave the letterbox bars. imho.
edit: you may have already known this info, i was just laying out the common frame or reference