In 3d because roads form boundaries the only way for two buildings on opposite sides of a road to connect is with a bridge. In 4d the ground would be a 3d surface so roads would not form boundaries like they do in 3d. In 4d two buildings on opposite sides of a road could be connected to each other without blocking off the road.
I was just thinking that perhaps in cities in 4d buildings would be interconnected so that it would be possible to walk from one side of a city to another without going outside. I mean perhaps the structure of a city in 4d would be similar to the structure of a sponge in 3d.