Update: Google says the Nexus 4 isn't sold out, but that the Play Store is just being inundated with demand. Here's what we're seeing on the Nexus 4 page right now:
Google Play is currently experiencing very high traffic. Nexus 4 is not sold out and will still be available for purchase. Please try again shortly. Thank you for your patience.
It's no secret that Google didn't handle the Nexus 4 launch as well as it should have.