Nexus 4: Size, or Generation?

So early this morning I was thinking about the name 'Nexus 4', then I got yo wondering, did Google name it '4' because it has a 4.7" screen, or because it's the fourth generation Nexus phone? Seeing as the Nexus 7, and Nexus 10 are named after their screen size, I'm just not sure. What do you guys think?