Reel,
99 code 250-32. If your building has more than one ckt (and I'm pretty sure we can assume it does) you have to have a grounding electrode (rod, water pipe, effectively grounded bldg steel, etc.) connected to your green, bare, whatever grounding conductor.
Using 4 conductors, your neutral and ground are kept separate at the out-building service, but the ground must have an electrode.
