I have the Japanese Laserdisc too, just for the additional scenes, which I refer to as the "International Cut", as this was also the version released in Europe. (On video cassette. Was there ever a PAL Laserdisc of Superman IV in any territory?)

I wonder, however, if the Full-Screen PAL VHS release will have superior resolution to the letterboxed Japan Laserdisc. The sides are cut off, of course, but perhaps one of you enterprising gents can line up the P&S footage with the LD Widescreen image and merge them for superior quality for at least the center part of the image. Not a simple feat, to be sure, but I've seen it done in at least one fan edit.

I suspect the "shrunken down" VHS PAL image within the letterbox frame will hold up quite well when bookended by, say, SD DVD footage.

Either way, in my opinion, due to the low resolution of the available sources for the International Cut, a SD DVD preservation will yield the most uniform results. The SD footage can be sourced from a downscale of the Blu-ray, and doesn't even have to come from the actual DVD. That way you preserve the colour timing of the Blu too.