Activate Auto backup.
Restart Zbrush to activate the functionality.
Load a tool.
-> crash
Deactivate the auto-backup and everything works fine
that’s what I suspected and what I wanted an answer for in my post. I take you are using win7 ?
Yup, I am.
Thank-you, guys! I was just about to post on this problem, but thought I’d check new posts first, to see if anyone had the same situation. And there you were! I just set that as a preference a few minutes ago and began having problems. Couldn’t save anything or open anything through Windows 7’s file system, only through Lightbox.
When I saw this post, I disabled the autorecovery feature and everything works fine again.
Good catch…
Although I do love ZB4 and highly appreciate what Pixologic is doing for us all who got this as a free upgrade, I have to say that the fact this issue isn’t fixed yet leaves me astounded to put it mildly.
actually it’s not even so much that it hasn’t been fixed, but that the option is still there in the menu as a living dead zombie, causing nothing but harm.
how hard can it be to have it automatically greyed out none optional on win 7?
to make matters worse it is not even mentioned in the new documentation which is just nuts. it is actually a feature of importance which everybody would of course enable right away to save himself from possible loss of hours of work.
I know many, including myself who pray for photoshop to have it one day and in 3ds max it has saved me more than just once from falling into a deep pit of madness 
but when the feature pretends to be available, but behind it hides exactly the opposite - a guaranteed or shall I say fail safe crash maker,- then that’s just slightly absurd and of a rather grim irony.
I can confirm the issue here as well. Win 7 Home x64. I just tested on my MacPro and it seems to be working there.