You have a project that's 16:9.
You have video you need to place into your production but it's 4:3.
Solution...
Find a photo or video-frame grab (16:9) that best suits your project.
Blur it.
Use that as a backdrop for your video.
Doing this retains the full frame of your video and can look really nifty if you choose the correct image.
A video, blurred, can also work.
Actually, you could take your Go-Pro video in the timeline.
Right-click to alter the aspect ratio.
Blur it.
Place your GoPro video (4:3) over (under) that.
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