I have been struggling with this for days, after all my problem was that I created a new account with a child birth date. Also I found it very un-intuitive that the gold + sign does not show at the second account. Anyway this is the way to do it first time right:
0. In case you have make the same mistake I made, by creating an account with child birth date, you should either create a totally new one (requires another email address) or contact Playstation and EA by phone to get the birth date modified...
1. Create new Playstation Network (PSN) Account (easiest is to do on the Playstation website but can also do on console.
Make sure you enter a parent birth date, otherwise the account will be a sub-account and you will not be able to link it with Origin/EA account later.
Activate the account in the PS4 by adding this account (log in with it).
Make your own PS4 the primary console.
Skip the purchase of PS+ (because you already have this on your main account). Your new account will allow you to make use of PS+ but it will not show through the gold + sign, don’t worry about that.
2. Create a new Origin Account (I did it in the Origin app but you can also do it via Origin website).
Also here, enter a parent birth date.
3. Link your new Origin account to the PSN account. I have read that this might happen automatically, but to be 100% sure I used the EA site (https://help.ea.com/).
Choose Login, don’t enter email/password here but choose the button in the bottom ‘sign in your account using PSN’. Here, enter your newly create PSN account.
Next you have to login with your newly created EA account credentials.
Now you get the message that your EA ID has been linked to your Playstation PSN account.
4. Restart the PS4 console, login with your new account, and you should be able to play EA games on-line using the PS+ online subscription of your main PSN account.
Have fun!
