Through reverse engineering of the 4R5 save routine and memory dump analysis (Userdump.dmp), the fault was isolated to the method.
The spinning wheel of death. The dreaded Windows "ZBrush has stopped working" dialog. The crash. zbrush 4r5 save crash fix
If none of the above solutions work, try reinstalling ZBrush 4R5. Through reverse engineering of the 4R5 save routine