Have you uninstalled 4R4 or 4R3 since installing 4R6? Uninstalling either of those will remove the C:/Users/public/Pixologic/ folder (or on Mac, the \Users\Shared\Pixologic folder). In that case it will need to be replaced.
In that case, see here: http://www.zbrushcentral.com/showthread.php?125810-GoZ-Update-1-with-Photoshop-Support&p=1030108&viewfull=1#post1030108
If you haven’t uninstalled an earlier version and the Pixologic folder is there then something got messed up in your permissions for the Pixologic folder and/or its contents. In that case you won’t need to copy the Pixologic folder over to replace it but you will need to adjust the permissions as mentioned in that post.