Hi
Depends whether it’s 16:9 letterbox or 16:9 Anamorphic. I presume it’s anamorphic as you are using the 16:9 settings in the composer window. This squeezes your stretched image to a true 16:9 image.
If your film is for broadcast check their delivery requirements. Broadcasters in the UK require that programs are delivered in the anamorphic format and they are then squeezed for transmission so if that’s where it’s going you don’t have anything to worry about.
If however your film is not for broadcast you will need to reformat it so that it is 16:9 letterboxed – there is an effect that does this in the REFORMAT section of the effect palette which squeezes your image and you can then put you composer settings back to 4:3.
I wouldn’t advise reformatting your image 4:3 as this cause you to lose a lot of picture information on either side.
Hope this helps
Jake Martin