So, I have two accounts, Canadian and USA. I used my USA account for Playstation plus and is my primary account in the system, my Playstation plus have expired. So, I'm planning on getting the card for my Canadian account, what do I have to do to use the Playstation plus on my USA account? Does I gotta change the primary account to the Canadian? is that possible? or just by having the account on the system it works?
This isn't how the 'primary' thing works. You have a primary 'console', not an account.
If the console is the primary for your USA account, then anyone on your ps4 will be able to use the USA's accounts games, and PS+ acccess.
If the console is the primary for your Canadian account, then anyone on your ps4 can use your Canadian accounts games and PS+ access.
They aren't exclusive of each other, that is both accounts can have this PS4 set as the primary.
This is how you make a system 'Primary':
The first time you sign in to PlayStation™Network, you normally select whether you want to activate the system. To activate the system after you first sign in, select [PlayStation™Network/Account Management] > [Activate as Your Primary PS4] > [Activate].
Do this for both accounts and you'll be set to use either :)