Hey there guys, I’m also having this exact same problem in Z4 and Z3.5 but only lately. Late last year I successfully decimated a 3 million poly model, now after clicking pre-process it ends up with the program not responding, even after giving it more than enough time to decimate. I’ve attempted all proposed solutions i’ve found throughout the forums over the past 3 days, Including aurick’s advice found in several threads. Listed below is all that i’ve tried to no avail, if your problem is as serious as mine, i hope the following info of mine will help you one way or another. I have still found no solution.
Specs:
- Intel Core 2 Quad, 2.4Ghz
- 8G Ram
- Win 7 x64 bit
When it worked:
-3.5 and 4 decimated fine
-Around (I think) 1 hour to decimate several million polys in Z4 (either way it DID work!)
What happens now:
- Both 3.5 and 4 runs fine until clicking on pre-processing. Then both crash.
- Pre-processing will TRY to run for several seconds, followed by the blue loading disc, followed by windows error of ‘this program is not responding’
What I’ve tried:
-Adjusting mem settings in preferences
-Re-downloading/installing Z4 in admin mode
-Re-downloading/installing decimation master plugin
-Giving propper permissions/ownership of all zbrush folders
-Uninstalling/Re-installing x64 and x86 C++ redistributables (control panel>programs) and redistributable found in zbrush troubleshooting folder
-Turning decimation master x64bit setting off (preferences)
-Defragmenting harddrive
-Attempting to decimate a 30k poly sphere-sits there for 5 hours-still crashes in both 3.5 and 4.
- Tried just leaving it all day to work itself.
- Tried all this again in admin mode.
Other noteworthy things:
-Looking at the memory used in task manager while decimating, the memory usage barely goes up at all, and then the program ‘does not respond’. E.g my mem settings in preferences will be 256 (default) and the program dosen’t even use that ram. Same story if mem settings are on 4096.
-I did find the 100 files in ‘vmem’ folder which apparently are used when you adjust that decimation % slider between 1-100% of decimation. (So the program is somehow doing some kind of work, but then it crashes?) 
-I did install an antivirus, however disabling/closing it changes nothing.
-I downloaded the Z4 trial again, along with plugins - same story.
Final thoughts:
I’m considering re-formatting my computer to start fresh. How can the programs not work properly when I haven’t fiddled with zbrush since last year? What then, has changed?