Updating to ZBrush 4

From ZBrush Info

Because ZBrush 4 is a stand-alone version, upgrading to 4 is the same process as for a fresh installation. ZBrush 3.5 (or 3.2 on the Mac) will remain an independent application and the upgrade process will create a new ZBrush 4 folder in your Pixologic folder. At any time ZBrush 3.5 or 3.2 can be used on your system after the upgrade to ZBrush 4. Do not run both 4 and 3.5 at the same time on any system.

Please, follow the Installation steps Installation, Upgrade and Activation