I solved snR84 level 4, then went back to try the GU challenge. Could not make progress, so fired up snR84 level 1 as a testbed. Did much better with clean start! So, tried to copy by hand from level1 to level 4. I know how error-prone this is, but it seems to me, level 1 solution does not work on level 4! But, I flipped two pairs at semi-random, and now my level4 is solved - but reports one GU less.
If you guys have any magic tools to compare my two solutions and report back, I’d appreciate it. If you find two pairs flipped, that’s what I know about. If you find a third pair mismatch, I apologize. If you *don’t* find a third-pair mismatch, then there is a bug!
ps - I have .jpg snaps of the two, if you need them.