A large store downtown has a heavy FOUR wire triplex feeding the
building. 3 wires are black insulated, the other is the bare neutral.

The thing that caught my attention to it, is that the 4th wire is not
connected to anything. All the triplex I've ever seen is 3 wire, two
are insulated, one is bare. That's for a standard 240V single phase
system.

Im thinking that this cable was intended to be for three phase wiring.
Three hot, and the neutral. Is this right?
Maybe that building once had 3 phase service, or they just had that
cable on hand when they wired it, and used what they had...