Thank you, Daniel. I copy that you are saying that the grounding conductor acts as the 4th conductor if it’s routed through an EMT pipe in the ground from the sub panel to the main panel.
Jim, can an EMT pipe be a metallic raceway? Does the EMT have to be 10 feet long or the same length as a solid copper grounding rod? How long does the metallic raceway have to go?
EMT is a metallic raceway. No minimum length, but it must run continuous between boxes or panel.
So both panels are located on separate structures. Therefore each panel if installed under a NEC code cycle prior to 2008 can be fed with only a 3-wire feeder if no other metallic paths exist between the structures and the neutral is bonded to the cabinet. From what I can see in the photo’s there is no metal raceways involved and the 2" is PVC. Each panel is required to be connected to a GES (grounding electrode system) at each structure.
No, Im saying the EMT pipe IS the 4th conductor. But only under certain conditions, like Robert said in the last post 
The main panel appeared to have a grounding conductor going through an EMT pipe into the ground below it.
The sub panel appeared to have a grounding conductor terminating at a grounding rod below it.
If the EMT is the 4th conductor, would you expect it to not have any conductors inside of it when it’s acting as a grounding conductor?
The two hots and the neutral would be Inside the EMT.
Are you sure that the conduit going into the ground is EMT? All I see in the photos is PVC .
I am not sure that the 3/4" conduit going out of the bottom of the main panel in the photograph is EMT. It does appear to only have a grounding conductor going into it. What it comes down to at this point is that I need some formal in-person training about electrical panels and grounding, as I’ve already taken all the other online InterNACHI classes on electrical inspections and I’m still in need of a better understanding of how this stuff works. Thank you @rmeier2, @dhorton2 and @jmilby for your assistance.
This stuff is very complex so don’t feel bad about asking questions, there are many experienced people here who are willing to help.
You’re welcome! I agree, the courses still left me a little wanting, and this forum was a huge help, from people like @rmeier2
The rest of the things you learn as you go, by noticing something doesn’t look right, and asking!