I would say, if there is ever a chance that you will have a parallel path, go with the 4 wire. If this is just a remote shed out in the yard that will never get connected in any way to the house, 3 is fine, perhaps even better since "ground" is a local thing.
edit to add;
If you are running 6ga and driving a rod you are actually making your GES better.
[This message has been edited by gfretwell (edited 12-27-2005).]
