"She lives on the fourth floor of this apartment building."
Translation:Elle habite au quatrième étage de cet immeuble.
Au rez-de-chaussée, au premier etage, au deuxième etage etc.
UK: on the ground floor, on the first floor, on the second floor etc.
USA: on the ground (or first) floor, on the second floor, on the third floor etc.
In Europe, including the UK, the first floor is the floor above the ground floor.