Most residential walls are 4.5" thick and interior walls seldom have insulation in them.
The 4th gen AEBS was known to have poor antennas but it should definitely work in a small apartment.
I used one in a 6,000'+ office space with metal studs and many walls for a few years before upgrading to the 6th gen AC model.
Also changing the channel your WiFi router uses to broadcast may get you a better overall signal.
I know in my stepsons apartment building you could come across a dozen or so WiFi networks. If most of these networks are all using the same channel to broadcast on, they will interfere with one another. Changing the channel can help quite a bit in some cases.