Let me clarify, I am not a lineman in any way shape or form. I am an 01 which is 600 volt and below commercial.
This is more what I think I see. See the image from Mike Holts website. The utility neutral is bonded to make a ground fault path to blow the fuse in a phase to ground fault. In the image Holt don't show the incoming utility wire very clear, but I see the ground/neutral bonded on the primary and secondary via the green dots. Look at the transformer in the image. Its a single black phase on top, and the return bottom has a green dot. Thats ground.
At my parents drop and other rural drops, a neutral/ground wire clearly travels with the phase on the poles. They don't have low voltage wires running those distances to carry the neutral/ground with the low voltage.
I was looking at much larger utility wires yesterday and they had 2 wires on top of the towers that ran unbroken with the insulated phase conductors. They even had a jumper going to a support cable in the earth.
At my home in Seattle, there is only 1 medium voltage wire on top of the pole, but I believe the load side low voltage is where they are maintaining the neutral/ground bond that goes back to the utility substation bonding the medium and low voltage together. So in a phase to ground fault say between your service mast and utiltiy wire, the fault would have to run many miles to a substation to loop back and blow the fuse on the pole mounted transformer.
This is a bit of an assumption. But I base it on I am very confident its AC on the medium voltage wire on the top of the pole at my Seattle home. AC has to have a return path. It can not work without it. From what I know, they only run DC on very high voltage transmissions lines up in the hundreds of thousands of volts going hundreds to thousands of miles.
They therefore then have to use the low voltage neutral/ground as a common neutral/ground with the medium voltage in urban environments. Since we are so dense, its less wire to do this in an urban setting. Therefore we don't see a second, or in your case, 4th wire with the medium voltage.
Thats my take. Where is a lineman. Don't we have one on this forum.