Rybka 4 will not be released with Convetka's "Aquarium 2010" as far as I can tell from a confusing array of ads and posts. For awhile it appeared the new Rybka would be out in early December, but now Vas Rajlich says:
"I've decided after all that we'll stick with KISS for the next set of Rybka releases. Rybka 4 will be released and sold when she is ready. Detailed information will be available at that time. Remote Rybka hosting will begin when that is ready. Further information will be available at that time. We'll follow some sort of soft launch protocol. I'd like to apologize to the guys at Convekta for changing my mind about several major things at the last minute, and to wish them luck with their upcoming Aquarium release...."
The Aquarium 2010 GUI appears to still be available with download releases starting tomorrow (accompanied by Rybka 3 or 2.3.2a, depending on which of the following options you choose). See this link for a brief description of the differences between the two options for purchase. If there are other options for purchase that you know about, please drop a comment.
I recommend both the Chessbase GUI and the Aquarium GUI. There are times when I want to use Aquarium's IDeA, which is a great feature (although it needs additional development for serious analysts, which may have been addressed), but there are other times when I want the clean lines of Chessbase (best seen now in the Fritz 12 GUI) and, as well, there are times when I want to use the Monte Carlo feature offered by Chesbase, which is now available in the Fritz 12 GUI (but only in conjunction with Rybka 3). If you're a Chessbase user, the cartoonish nature of Aquarium will take some getting used to, although I haven't seen Aquarium 2010 yet. If you don't want to go to the expense of both GUIs, then decide which feature means more to you: IDeA or Monte Carlo.