Four are the familiar main operational stations, two are cargo transporters
...So, do we see a cargo transporter in "Dagger of the Mind" where only cargo is supposedly being handled, and the stowaway within it ends up somewhere near Deck 14? Or would that be one of the four personnel transporters, as it's identical to them in design?
As I recall, the "cargo transporter" in "Dagger of the Mind" is more or less identical to the personnel stage in virtually every episode.
This raises an interesting question: the STTNG Tech Manual states that cargo transporters are defaulted to a molecular resolution setting, instead of the quantum resolution that personnel transporters must operate on. One must assume the transporters on the 1701 operate in the same fashion. Unbeknownst to our heroes, Simon van Gelder is hiding in the containers meant for the Bureau of Penology beamed up from Tantalus on the cargo transporter. It seems to me he's lucky not to have materialized as 90 kilograms of wet hamburger.