For SWAT 4 players:

If you guys wanna hook up on a private server you need to install a Virtual LAN software, otherwise keep reading.

If you have a fresh install of SWAT 4, make sure to install the server patches from here:
https://github.com/sergeii/swat-patches/tree/master/swat4stats-masterserver
Under the "How to install this patch?" section you are looking for "SWAT 4: Gold Edition" which has two patches. This patch allows us to connect to a new master server to which we can connect to from the Multiplayer>Join Game menu.