The best way to have a clean 4k would be to... use the 4k master the "Institut Lumière" did in 2015 for the 120th anniversary
The "original" link in the colored version is misleading. It is NOT the original used to upscale to 4k. The right link is in the B&W 4K/60p video before colors.
They upscaled the 1080p from the bluray "Lumière ! L'aventure commence" (that itself has been done with the new 4k master!)
Uncropped YT version : https://www.youtube.com/watch?v=ymjlHsPmesk
Trailer with english subtitles here : https://www.youtube.com/watch?v=BVcRXIN_woc (it seems it's on Amazon Prime in some countries (I don't have it here in France))
I really don't like the 4k/60p upscale... I think it's worse than the clean 1080p original. I don't see any interesting details. I feel like it mostly adds ugly artefacts everywhere.
The AI part is often just a semi-bullshit marketing term. Topaz Gigapixel AI is just another commercial "AI" upscaler. Everybody has one nowadays. Even my new cheap tiny TV box thingy (Shield TV) has an integrated 4K upscale with an "AI enhanced" label on it. I'm not sure where the AI comes in that. I'm pretty sure it doesn't use any neural network/tensors/anything in realtime and doesn't learn when I feed it videos
For the 60p: it doesn't help where it could (where the original "jumps" sometimes).
And it's badly cropped.
Colorization with DeOldify is still impressive but still far from perfect. It still has those colors leaks, and was clearly not made for videos.
That's not much than a fun experiment, for me