4 backs, I'm curious how you came to that number. I can tell you that I won't even consider going out of the studio with less than 5-6 backs for anything serious. Substitute a Dicomed FieldPro for one of those from time to time, but still, 4 backs?????
I think Paul Ron works on backs and bodies as well as lenses, drop him an email, he's a user here, so a quick search should find his info.