This one is a much larger bugfix/balance release. The asteroid seeding is a lot better now (only in new campaigns or savegames being upgraded to include asteroids for the first time, though), and there are a couple of new cheats (handy for us for testing), and the core shield generators have a few more kinks worked out of them.
Also, the wave size scaling by tech level has been adjusted in a nontrivial fashion, and a huge number of data centers are no longer seeded (as well as being cut out of existing savegames that are upgraded). These changes are also intended to provide more fluid, less exploity-if-you-do-the-right-thing play.
And, of course, a dozen or two of other miscellaneous items.
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.