If you want continue to play and join servers for a challenge with other players, you have to use Tunngle.
Tunngle is like Hamachi, but what is hamachi ? hamachi (and tunngle) permits to communicate with other peoples as you were in the same LAN. It's a network in the network (a private network in Internet).
So, why tunngle ? because tunngle provide a powerful rooms system and they created a room for each games. With this system, you just need to enter in the SWAT4 Room to display availables servers and join one.
When you have a server, this one must be running as "LAN"/"Local" server if you give it available on the tunngle network.
What shall I do ? Just install tunngle, the last version is available with the link : http://www.tunngle.net/
For more informations about tunngle visit wikipedia : http://en.wikipedia.org/wiki/Tunngle