I tried to sketch a map based on the description, but my sketch figuratively crashed and burned: If the 80'-hallway is "flanked" by two 10'-wide hallways, isn't the whole length 80+10+10 = 100'?
Keeping it at 80' overall left me putting the fourth door within the width of the eastern 10'-wide hallway that goes to the south, thus:
Door 1 in the north wall of the east-west hallway is 5' wide; it's 35' from there to the antechamber; the southern door of the antechamber is 5' wide; door 4 is 30' farther to the east, and door 4 is 5' wide. That's the full 80' of the length of the hallway--but door 4 in on the south side of the east-west hallway, right where the 10-wide Branch 3 is.
I think I misunderstood the description.