Even with demarcs, you are going to have metallic paths! If both buildings have the smae phone line - now or in the future- they will have a common path. Cable will always have a common path, that's also bonded to ground at each building via the shield.
The electrons don't care if they are going through a demarc or not.
4 wire is the only way to go with something like this. Even if you can prove there are no mettalic paths now, is the gas man going to repull wires when they or a future resident install gas down the road?
