This one is the third release candidate, and is 7.0, unless some fairly critical bug or other such issue comes to our attention over the rest of the weekend.
Actually the changes in this were not for critical issues, but there were a couple very simple (as in just changing a couple boolean flags) changes that make the Zenith Hydra Head spawnable work a lot more sensibly. I used to be able to leave well enough alone, so I blame Chris's example.
This is a standard update that you can download through the in-game updater itself, if you already have 4.000 or later. When you launch the game, you'll see the notice of the update having been found if you're connected to the Internet at the time. If you don't have 4.000 or later, you can download that here.