This is the 1546th most frequent Polish word.
czwartym
"Fourth" (in the instrumental or locative case).
Here, czwartym is used to describe the order of the floor within a building.
To zdarzyło się na czwartym piętrze.
It happened on the fourth floor.
Czwartym again acts as an ordinal adjective, indicating the sequence of the language.
Rozmawiałem w czwartym języku, którego się uczę.
I spoke in the fourth language that I am learning.
Czwartym is an ordinal adjective here, modifying the noun 'room' to indicate its order.