The PSN is Sony's online gaming community network, and you need to be subscribed with PS+ to get access to this. You logon to this via the user settings for your PS4 user profile. In the System Parameter menu on your PS4, you have a test function that you also can run, which will tell if you are online and recognized by the Sony PSN network.
Next, when you have started your BF4 program on the PS4, you need to setup your soldier to match the account you have created at EA.com online using an internet browser (via smartphone, iPad or PC). Here you also have to use exact and correct soldier name, matching the email address and password you have setup with EA. You also within the BF4 game on your PS4 have a testing menu, to check your connectivity to the EA server network for the BF4 game servers. (Within BF4, go to Options, and here you can press R3 on your PS4 gamepad, which will start a system test...)
When both of these pairs of information has been entered on your PS4 with your BF4 game, then you will be able to go into the multiplayer online gaming universe.
