From TFA:R thread:
And no, none of my edits will ever surpass 1080p BluRay spec for a host of reasons.
If anyone would ever like to attempt it, feel free to lay my lossless audio output and a proxy rip of the video streams into an editor and recreate the video in 4K or whatever. But I cannot, due to technical limitations and much of the source material being sourced from 1080p and 480p. The entire saga is being mastered at 1080p with 5.1 DTS and that’s as far as I can take things.