I added a drawing. of both wiring configurations. The only advantage I can see is the 4 wire gives you a dedicated ground fault circuit vs sharing the neutral wire. Open neutral between the two panels would make the ground hot in the 3 wire.
Open neutral between the main panel and transformer would make the ground hot in the 4 wire.
I am trying to understand how this works and why the code changed. In this case you have a detached shop 250 feet away with no plumbing or other connections to the house with the main panel.
The current NEC code requires a 4-wire feeder so there is a separate grounding connector between the Main and Sub panels. The neutral and ground are not bonded in the subpanel. In this setup if a hot wire coming in contact with the non-current carrying parts of the electrical system, (outlet covers, panel covers etc), the 4th ground conductor will provide a low resistance patch back to the source tripping a breaker. This 4th grounding connector ties the ground rod at the sub panel to the ground rod at the Main panel making a grounding system.
How were the sub panels wired when 3 wires were allowed? Was the neutral and ground bonded in the Sub panel so the neutral wire served as the grounding connector? Was this code changed for the same reason 220 volt plugs went from 3 to 4 wire?