What is interesting is that in theory, your IP address should change every couple of days or so. Your ISP uses DHCP to assign you an IP address. This address is leased to you. ISP's have different time frames for leased IPs to expire. Generally this is every 2 or 3 days. Now here is the rub.... Most ISPs now will attempt to renew your IP with your existing IP when the lease expires. So, it is not uncommon for a user to keep the same IP address for years. The whole reason for having leased IPs is to prevent users from using their home computer as an internet host. I guess ISP's have gotten lax with their policies on this. Having said that, I can easily release my current ip lease, and then renew a new ip. If I were going to steal from someone on the internet, I could very easily spoof my ip address and take other measures to hide my identity, but the average user would not know how to do this.

My whole point of this, is that if FP has found that there are 4 users that are consistently using the same ip address via log files that he has access to, then these 4 users are indeed the same person to a certainty (or at the minimum using the same computer).