I have a customer with a pole mounted 400 amp service. From the meter enclosure, a 400 amp disco is fed thru a gutter. This is all grouped on a board at the pole. From the disco there are 3 500MCM which go underground about 100 feet to the main residence, where there is a 150A disco and MDP. Now they are building a shop which will have 100 amps run to it, also from the pole mounted service. The shop will probably have metallic paths to the main residence, but not to the pole mounted service.
Question is, 3 wire or 4 to the shop? I feel 4 are required since in the event of an open neutral, the current would attempt to return thru the metal path to the main house where it would find the neutral at the bond there. Also, if the main house experienced an open neutral, the current would return thru the metal path to the shop then on to the service. I am 99% sure on this but would like some confirmation.

(P.S. The metallic path would be phone/cable. The water will be PVC and no gas)