True, you can do this from the web page or psn app as well ,
if you have more than one ps4 or PS3 or other on your network one requires to be primary to deal with the psn / and psn plus network features , and the other devices will use the psn/psn on the primary decive.
reasions why that could have happon , if u have got a replacement ps4 from Sony
that the information stored at Sony and stored on your device has become mismatched
i don't know but if you re initialize your ps4 it may require to be set as primary again.
or someone has been playing around with your psn account ;)
yes .... if in dout always keep changing your psn password , and keep it safe to your eyes only.
sony is now offering , text message security feature for your psn account , for people that can't keep there password safe :)