See, the issue is the Concourse Level
was on the same level as the street on the north, south and west sides of the complex.
all of the buildings shared had a floor that was the same level as the concourse, but the concourse wasn't entirely part of them, they each had at least a lobby section on that level was wasn't part of the concourse, even if they were at the same elevation.
1, 2, 4, and 5 all had lobbies that opened into the concourse, but were not part of the concourse. 3 and 6 had floors at the same level as the concourse, but didn't connect to the concourse.
To give you a better idea of the floor count of 4, 5 and 6 WTC from the lobby levels at the street to the top floor I took some pictures of my WTC model and labeled them. Hope this helps you.
4 WTC, 9 floors
5 WTC, 9 floors
6 WTC, 8 floors
