How to fix NP-31730-4

1. Go to Settings > Account Management and sign out from PSN. Then sign in again.
2. Turn off and unplug your console. 
3. Wait 5 minutes, then plug the console back in and restart it.

If you're still having issues, try backing up your data and resetting your console.

1. Go to Settings > Application Saved Data Management and back up your console data.
2. Once you've created a backup, go to Settings > Initialization > Initialize PS4.