In the past, the previous nexus has disappeared when the new one come out.
Also, I think the nexus 4 is mostly sold out. They tend to stop selling the old one a few weeks before the new one is announced. If you want the nexus 4 you're going to have to act on that fairly quickly.
If this nexus follows past nexiii, you can expect it to be in that $300-$400 price range off contract depending on storage options. I haven't seen anyone saying anything differently yet