We had someone here at work do something funny to his PC and it though it was a print server and this PC was giving out like 2 GB's a data at least a day.
How did you determine these 4 PCs were the culprits? You had to scan the network for traffic to do so. What kind of traffic were these machines spewing?
Plus I'm assuming you took them all offline and are troubleshooting them and reintroducing them to the network one at a time with careful monitoring, right?