There are 6 bolts on a cb550 hub to attach disk, I'd geuss that cb400/4 whhels have more in common with cb250 wheels from the sound of it (disk diameter and offset?). Keep looking and some correct cb550 wheels may well surface. Don't be put off by cracked rear hub liners though, it's a bit of a pain but not inpossible to put in new linings if you have a lathe or someone in huddersfield re-lines them for