Your Compact Mem setting is correct for the amount of RAM that you have. You should make sure that your Undo settings are left at the default of 4. Max Polys Per Mesh should be no higher than 12, but a value of 6 is ideal if you’re working with ZAppLink.
4 GB of RAM on a 32-bit system would help when you’re using ZAppLink. 32-bit versions of Windows will not give more than 2 GB of RAM to any given application. But if you have 4 GB you would be able to set Compact Mem up to 2000, giving ZBrush the maximum amount of RAM under a 32-bit OS. The remaining 2 GB would be left over for Windows itself, Photoshop, and any other apps you’re running. This will make a difference when working with large files in ZBrush.