In order to play a TCP/IP game, all players that are behind routers must be able to forward the necessary ports. It will not work otherwise.
The "source" IP address is just the IP address of your computer. This is either detected as your WAN IP if you have no network, or as your internal network LAN IP address. In either case, SE4 will work fine over the internet, after you have forwarded the relevant ports. It would make no sense at all to be able to change this IP address. If you did, you would not be able to connect with the game, as it would try to use an address that does not work...
TCP/IP is very slow for more than 2 players though... SE4 will only send the turn files from the host to a single player at a time, irregardless of connection speeds of the host and the players involved. It is generally much, much faster to use PBEM or PBW (
http://www.pbw.cc ), or even a FTP server (which anyone that has access to their router config for forwarding a few ports can easily run with
Filezilla Server; make sure to enable passive mode on it and for client FTP programs) if the host has a good upload speed.