This is the 4413th most frequent Japanese word.
四つ
"Four" or "four things."
Used as a counter for objects, indicating the number four.
私は四つのみかんを買いました。
I bought four oranges.
Used as a counter to specify the quantity of possible responses.
その問題には四つの答えがあります。
That problem has four answers.
Represents an age, specifically four years old.