Both myself and my boyfriend have PS4s.
Can I just get a single PSN account and sign both consoles into the same account, or do I need to get two separate accounts?
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So based on how you plan to do this, the answer is No and Yes.
From About Activation:
How many devices can I activate on one SEN account?
The number of devices that can be activated on your account is limited for each type of content. For example, if you have two PlayStation 3 systems activated for games content, you cannot sign-in on a third system with your PlayStation Network account information and download and play a game. You would need to deactivate one of the first two PlayStation 3 systems to allow you to activate the third.
So you'll need two separate accounts, as each one will be assigned a PS4 to it.
Now in regards with just having one account to share, that's possible too. From the same page:
On how many SEN accounts can one device be activated?
You can activate a PlayStation system on as many SEN accounts as the highest number of possible local user accounts on the system.
This means that you can activate a PlayStation 4 or PlayStation 3 on up to 16 SEN accounts at once and you can activate a PS Vita, PSP or PS TV on one account at any one time.
There's a link on the page as well to explain how to share your account's content with your "friend's" console:
Create a new Local User account on the PlayStation 4 and go to [Settings] > [PlayStation Network] > [Sign in]. Note that system parental control settings may restrict this option.
Sign-in to your SEN account.
Select ‘No’ when you are asked if you want to activate this PlayStation system as your ‘Primary PS4’. This is to ensure you are able to continue to access your PlayStation Network content on your own PlayStation 4.
You and your friend now have temporary access to your PlayStation Network content from your friend’s system.
Using these steps, you can make the first account, tie it to the first PS4 and buy the majority of games onto it. Make the second account, tie it to the second PS4, log-in and save the first account's credentials to the second PS4 as well to share those games between the two systems.