I cannot connect to someone as a client / someone cannot connect to me when I'm host / the game disconnects soon after starting or in the team lineup screen! Help!
If this happens, then you are almost certainly using some sort of router or firewall. Pro Evolution Soccer 4 requires UDP port 5739 to be open for online play to work as a host, and the entire UDP range needs to be open to play a client. Please read through the guide carefully to set up and configure them correctly. Unfortunately, for some users of a router, they can still only play the game as a host even after configuring everything correctly. The only solution after that is to plug the internet connection directly into your computer and use a software firewall.