Most people might not know or care about what it means to have a primary PS4, but it can be really important. Each PSN account can have 1 single PS4 console set as the primary system. When you download games from the PS store to your primary PS4, all other user accounts (using different PSN accounts) will be able to play the games that you purchased with your PSN account.
If you set up another account on a different PS4 that is not your primary PS4, you will be able to download and play games, but only when you are logged into your PSN account. If you switch to a different user account, the games will say something about there being issues with the license.
Chances are you set the PS4 as primary when you first set things up and never noticed it or thought about it again. Here is what you need to do to set a system as your primary PS4.
Set A System As Your Primary PS4
Load up the PS4 that you want to set as your primary PS4. Log into the user account you want to set the primary PS4 for. Go to the settings option in the main menu and select the following options. Account Management -> Activate as Your Primary PS4 -> Activate.
The PS4 you are using is now the primary PS4 for this users PSN account and any other systems will now be deactivated as the primary system.