OpenChess

houdini 4 is out now
http://www.cruxis.com/chess/houdini.htm

+50 ELO from houdini 3

It seems that the manual is about half-updated.
http://www.cruxis.com/chess/manual/index.html
For instance it still says "Houdini 3" and quotes Anand as being the "current" World Champion in the Introduction, and the Version History is still v3. Also the pictures on the "Configuration" page have v3 screenshots. On the other hand, it does give a lot information about the (new) Syzygy capacities. On the slightly annoying side, at least for me the "2. Installing and Running Houdini" link on the left does not work (I think it did previously). After a bit of clkcing around, I got the first 3 icons in the left-side TOC to expand, and then I could access the pages. But just clicking on the major section 2 does not do anything (unlike major sections 1 and 3). It seems that the instructions in 2.3 are indeed for v4 and use the screenshots from it. As I say, the manual overall seems to be about half-updated.

I got the following clarification from Robert Houdart:

"The online User’s Manual is exactly the same as the PDF and CHM help files provided with Houdini 4 (installed in C:\Program Files\Houdini 4 Chess).
Inconsistencies in the online version are due to browser or proxy caching, it usually helps to refresh the right side of the page (right-click on the page and ask for a Refresh of the frame)."

This is indeed correct, I was able to get the newer information via reloading frames. I have actually cited the Houdini manual in a paper I co-wrote (particularly its calibration of scores), so maybe I should look for a more permanent URL for that purpose (the paper is nearing the final proofs stage).

I still note, however, that the pictures on the Configuration page seem to be Houdini 3 screenshots (Scorpio rather than Syzygy).