There's no such thing as friend codes for the ps4. I can tell you it looks much better on ps4, basically the same as a PC with average specs. If you have a higher end PC it will play the game better though (Hook your PC up to a TV and use an xbox controller for the couch experience, or get the Alienware Alpha console which will also certainly run the game better than the PS4). The good news with the ps4 though is it has none of hte issues with some graphics not showing up or popping in etc. You can also use a mouse and keyboard on the PS4 if that's your thing.
There is a recruit a friend program but I'm not sure if you can benefit from that if you already have an account (edit, I checked and you have to create the account using the code provided by the friend)
Personally, unless you've already leveled something up to the max I'd just create a new account for the recruitment rewards.
To fix your issue go to the mog station and log in with your square enix account and re-activate your service account (Not your subscription). In the future if you quit, cancel your sub instead of your service account. I really dont know why its designed like that but then again this is SE
