It is not a matter of 32-bit. ZBrush is still a 32-bit application. It is a matter of the OS.
Fibers, MicroMesh and other ZBrush features push your system’s hardware. They push it hard. ZBrush communicates with your hardware through the operating system. In order to tune ZBrush to make efficient (and stable) use of your system’s hardware requires a seamless communication with the operating system.
As programs evolve, they gain new features. Eventually it reaches a point where the new features outstrip an older operating system’s ability to support them. ZBrush has during its lifetime discontinued support for Windows ME, Windows 2000 and Windows 98 – each time when it reached a point that it was no longer possible to advance ZBrush’s feature set while still maintaining support for the earlier operating systems. Eventually, Vista and 7 will go the way of the dodo as well.
Microsoft discontinued XP in 2008. They started phasing out support in 2009 when they moved it to their “Legacy Support” category. As of April of that year they stopped developing any new functionality for XP, including development of features that software developers needed for their applications. While Microsoft continues to release monthly updates for XP, these are for security fixes only. Not only are there no longer design changes, but warranty claims and free technical support have also been discontinued for the last four years.
While we would have liked to continue supporting XP, ZBrush 4R2b is just too unstable under that OS.
ZBrush’s current system requirements are listed here: http://www.pixologic.com/zbrush/system/ The only thing that has changed from 4R2 is the removal of XP from the list of minimum requirements. Since your hardware has certainly not downgraded between 4R2 and now, that means that all you need to do to run 4R2b is upgrade your operating system to Windows Vista or Windows 7. That does NOT mean upgrading to a 64-bit version if you don’t want to or if your CPU is not 64-bit bit.
You can of course continue to use ZBrush 4R2. You simply will not be able to use the newest versions any longer.