Do outbuildings require 4 wire service to a sub-panel
In every part of the country, any subpanel requires a 4-wire feed. That's been code for well over a decade.  

Additionally, any outbuilding (not connected by a breezeway) requires a Grounding Electrode System of its very own.  

The reason is  because the wire takes human-made current back to source, but the ground rod takes nature-made current back to source.  Dirt doesn't conduct well enough to do both. 

Retrofitting ground isn't that big a deal

In a 3-wire outdoor subpanel feed, you can just add the ground wire.  It needs to get from A to B, it is not required to run the same route or lay in the same trench.  

Rigid Metal Conduit or IMC only needs a 6" burial depth (but it's expensive), and it qualifies as the ground path. Lay a 1/2" steel pipe and yer done.  Use it for comms later :)