Topic: How do licenses work if you have two PS4s?

Howdy folks.

Pretend I'm a massive idiot when it comes to video game hardware, and I know absolutely nothing about how the systems work.

Now, my girlfriend and I are both gamers and have two TVs set up in the living room. There's a PS4 under one TV and a PS3 under the other. More often than not I'm on the PS4, while she's on the PS3 catching up on all the games she missed last gen since she had a 360.

Now she wants a PS4 and I'm wondering about how the licenses work. I buy almost all of my games digitally. I know that she can play my games when she's signed in on my PS4 because she's done it loads of times. But if I have a second PS4 how does that work? If it's not my primary console, presumably she won't be able to play my digital games on that one too or people would surely just exploit that by signing in on their friends consoles once and letting them download all of their games.

If someone could clarify in simple terms for me how all this crazy technology works I'd be eternally grateful.

@johncalmc This is actually against Sony's user agreement and thus really can't discuss it here. You would have to Google it I am afraid.