I was wondering if there are going to be any problems opening ztools in Zbrush 4 that were made in Zbrush 3.5 R3. Maybe some of the version 4 beta testers can answer this one.
Beta testers are still under NDA. They can’t say anything. Please don’t ask beta testers to break the rules. 
Pixologic always tries to ensure backward compatibility with elements created in older versions. There are cases where this is impossible, however. For example, when we overhauled ZSpheres in the past this forced ZSphere models from earlier models to no longer skin the same.
But as a general rule, you can open stuff from earlier versions in a newer version. You just can’t go the other way. When you encounter an exception you can always export from the earlier version as an OBJ and then bring it into the newer version that way.
Thanks for replying and answering my question Aurick and sorry, I didn’t realize the beta testers were still under a NDA. My bad! Say no more. Nudge nudge, wink wink!