After installing this torrent /torrent/7992789/Zbrush_4r5 I was having the same problem as the other commenters... keygen seemed to work, but after using the program for 15 or 20 minutes I'd get a popup saying it couldn't save and it would quit to desktop.

I looked around for a fix that supposedly existed and found this file.  Tested it out and was able to use the program and save my progress.

1) install Zbrush 4r5 full (MD5: 478c0ab9b8f7cdfe084d4ec685c0a90f)
2) activate using xforce keygen
3) copy Patch4r5.exe to same folder as ZBrush.exe
4) run Patch4r5.exe

enjoy!