This is the information I have learned after looking into this:
1. The FTP server is sitting behind a NAT and sees all incoming connections arriving from the same IP address. Therefore...you just happened to be the fourth connection using the same IP address.
2. Other people from YOUR network are accessing the same FTP server. Since you all may appear to be coming from the same IP address...
3. In a program like filezilla you can select File -> Site Manager On the Transfer Settings tab, set the Limit number of simultaneous connections field to, in your case, less than 4.