Hello philwizzer123,

- stretching a 4:3 to 16:9 ?

Of course I don't know what your 4:3 clip is about, but it will be rather distorted......?
Why not use a colorboard / image / videoclip behind the 4:3 instead?
Or maybe as a smaller PiP? ( rezise the 4:3 in Modify keeping the ratio)

Nina
Just something.
https://www.petitpoisvideo.com

Thank you Nina
Primarily I am looking simply to fill the project screen rather than having two black or coloured bars down either side. However your suggestions are good ones and perhaps that's a more 'artistic' way forward.
Thanks

Nina is great, isn't she?
In the past, I have taken a screenshot or frame-grab of the video in question, or another image that relates to the video content in some way, blur the image, and use that as my background, even with a bit of Magic Motion.
This eliminates empty black bars, looks more interesting giving your production a jazzy layered look, and because you keep the 4:3 video as-is, you'll not lose so much detail.
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https://www.facebook.com/BarryAFTT

That is exactly what Broadcast TV is doing with 4X3 video today on news programs.

An example of what BarryTheCrab said.
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Thanks to everyone for their suggestions
philwizzer