I recall how they started the first “The Incredibles” movie: it’s documentary interview footage meant to set up what their past was like. To get across the time difference, the doc footage was 4:3 matted into the center of a widescreen frame. They do a slow push to make it bigger, but they always keep it centered with lots of dead space around it. When they then smash-cut to more modern times, they go full- frame.
A concept I like in mixing old 4×3 footage into 16×9 is to project it onto an iconic object, something that references back to the period, and *that* object is shot in widescreen. Could be the side of the building where it happened. Could be a still life of related prop objects, like clothing, personal effects, etc….
A very typical version of that is to composite the old stuff onto an old 4×3 TV set on a limbo backdrop, and then fly 16×9 camera moves around that TV to punctuate and add emphasis. Makes a fun device to cover edits. You can get more creative yet, and composite the shots onto the monitors of a TV Director’s live-switch room, as well as the viewfinders of studio cameras, with a reproduction of the original set on the stage, perhaps. So you’re like a ghost walking around the production as if it was happening in real-time, but only visible in those monitors and viewfinders.
You can do something similar by setting up an old projector and movie screen in a smoky dark room, then compositing your old footage into that screen, and again moving your camera around this framing device.
Yet another method that doesn’t involve pan-and-scan is to to make a multi-matted 16×9 composition, putting your 4×3 stuff in one side and then adding changing trains of dissolving stills in boxes around it. Change the positions and arrangement of the boxes from time to time but keep using the rule of thirds and golden mean in creating the arrangements.
These are creative ways to avoid using the pan and scan blow-up or the matted duplicate blurred clip as background.
I run a youtube channel and some people that send me vids give me vertical video. 9 and agita, obviously). I found a public domain shot of a hand holding an iPhone, and I composite their vertical video into that 16×9 frame to show what they sent me, and I like that a lot more than the blurred sidewalls thing. You might consider THAT for your 4×3, with the conceit being that your clips are all on a web site you made up.