I understand the part that you can login to any psn account and play as that person - cool.
I also get the part that you can login on any PS4 and play your account.

Trying to get my head around the whole primary account thing. If my boy has his primary account on my console and vice versa. Lets say I bought Fifa14 on my account would we be able to play against each other online with both consoles - much the way you can with 2 x PS3?

Should be able to yes.

Essentially that's all I want to do.
Have a laugh with the boy and in the process possibly knock out some idiotic online trophies at the same time!