I was lucky enough to score a set of NOS Hooker 4-into-4 slip-ons a few years back for my '76 CB550K. I'm sure these are unobtanium at this point, but I think similar set could be conjured up using four "generic" reverse cone megaphone mufflers such as offered by JCWhitney and the like. You would have to fab some custom angled adapter tubes to mate with the headpipes, and some custom hanger brackets for the megaphones themselves.