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Each domain in a PBN should leave no footprint and if you have found it then Google will find it/them. For the 4 sites have you got any proof of relationship - contact details, whois, duplicate content, same plugins/layout, a very good reason to believe they are from the same source? It would be very foolish of them to leave such a huge footprint as to link to all the sites. Have you checked the PBN domains to see if they share the same C class? e.g. http://smallseotools.com/class-c-ip-checker/? or http://www.authoritydomains.com/bulk-ip-checker.php If they were totally lazy then there is probably only a handful of hosting accounts.  If that was proven then I wouldn't have any difficulty talking to Google if the SERPs were being distorted by more than one site dominating but basically from the same source.  If it was a single site i'd tend to sit back and watch, while building a strong link profile.

Good point EGOL on showing that after you crush your main market there is actually an opportunity to produce secondary sites, I hadn't thought of it in that way before.