You will need a dedicated server to host the game. I had the same problem with only 3 or 4 inputs synced, it appears they eat the bandwidth and make the game unplayable.
For turn based games it would be ok to have 1 player as the host. From what ive seen though, using this method for real-time games is not possible.
C2 multiplayer using peer to peer connect , why i need a dedicated server to host the game???