First sorry Chickenman for my rather unfriendly sentence at the beginning of my post, I was in a bad mood. - no offence intended.
Well I still have to say that it is a mistake to use the term "aspect ratio" when it comes to 16/9 - 4/3, specially for DVDs.
Why? Because both 16/9 and 4/3 formats, on DVDs, will actually fill a single 1.33 aspect ratio image anyway! There is NO other "aspect ratio" when it comes to the image actually recorded. A 16/9 image is only a widescreen image horizontally shrinked to fill the 1:33 frame, then this image will be stretched during playback, either by the DVD player or the TV set, so to restore the widescreen.
But in both cases, the raw aspect ratio of the image recorded, before stretching, still is 1:33!!
That's why I state that 16/9 and 4/3 don't refer to aspect ratios, but are two video formats/systems to manage the aspect ratios of recorded pictures.
I hope that I make myself clear enough so you can understand that I'm actually discussing a point of terminology.
This is a rather complicated subject and is difficult to explain only with words...
I found this interesting page http://www.hometheaterinfo.com/video1.htm but sadly there are no drawings.
update found this page too, more "in-depth" and with drawings: http://members.aol.com/ajaynejr/vidwide.htm
Now to the question from Murray and (or?) Rapallo, now I get it :), but sadly I can't help simply because I do'nt know any software that will perform a high quality zoom/resampling of a 4/3 letterboxed image, then shrink it horizontally with the proper algorithms to fill a 1:33 frame that will be recorded to DVD... put in other words, transform a 4/3 image into a 16/9 image without changing the aspect ratio.
Maybe such a software exists that I'm not aware of. I'm very interested if someone knows such a software, even if I guess that the whole process would use huge processing power and take ages, unless a dedicated chip handles the task...
Personally, I can live with the resolution loss from zooming a 4/3 widescreen picture, because my 16/9 TV set is not huge. But I guess that this would indeed bug me if, like Rapallo, I used a projector...