When you select 4:3 and 16:9 in ConvertXtoDVD, your only telling it what kind of TV you will be watching it on.
It’s real important to check the Aspect Ratio of the input file.
If it’s not a true 4:3 or a true 16:9 it’s absolutly normal to get some black bars on top/bottom, on the sides etc…
Now loosing some video on each sides, maybe caused by “Overscan” or CRT TV, which VSO can’t do much about !
Google OverScan !