There's no question it's there but is it a joke or a tease just like the newspaper article in UC3?

Where's this??

In UC4.

This has got to be a hint. What chapter is this in? 

"American Daughters" as a subname, hmmm not sure what to think of that but still, it's a good sign it's happening.

I can't say, that would be a spoiler for people who haven't played it yet.

Lol ok. It looks like that's the dark horse logo on the corner so this could be a comic book series that's coming. Also I didn't realise when first looking book that female is pregnant, so it could be a prequel comic and that's Ellie's Mother. Was t it her death that caused the uprising of the fireflies? 

Naughty Dog just teaseing :p

Would be awesome with exactly that title and cover art.

I freaked out when I saw this, but I think this is actually a teaser for a new comic (notice the Darkhorse comics logo).
The resemblance to Ellie is uncanny, but why would she wear a gas mask if she's immune? Perhaps for her daughter?
Also, considering Ellie is a lesbian, this doesn't make too much sense.
It would make more sense that this is a prequel surrounding Ellie's mother.

After Left Behind, they specifically dismissed Ellie being a straight up lesbian, she was obviously very close to Riley but it didn't define her sexuality.

I think it's a nice easter egg, I got a buzz seing it. First thing I thought was Ellie doesn't need a gas mask, then I noticed oh, she's pregnant, lol. Maybe she needs it to protect the baby.

I don't know if they'd just straight up tease Last of Us 2 like this, either it's just a hint that we're getting another one, or as people have said it's some kind of comic/non-game content.