Were you delivering mail or packages? Because apartment complexes around here usually have a set of letterboxes just inside or even outside the building, so the mailman doesn't have to climb to the 4th floor.
Sweden built a ton of housing (the goal was a million apartments in 10 years or so) back in the 50s and 60s. Those buildings didn't have letterboxes inside, all with letter gaps in the doors (at least where I worked). Newer places typically have them downstairs now.
The houses themselves are pretty good actually, but sadly due to where they are and the amount of them many have become undesirable neighbourhoods.