This one has a number of balance tweaks most specifically centered around the various ion-based units and the spirecraft attritioner. These should now be a lot more fearsome in AI hands, but even more notably a lot more attractive for players to actually build and use themselves. There are a couple of other minor tweaks and fixes in there, too.
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.