The fourth day of the month (a calendar date). Read as 'yokka' in Japanese; often written 四日.
今日は四日です。
Today is the fourth (of the month).
来月の４日は祝日だから会社は休みです。
The 4th of next month is a holiday, so the company is closed.
申込みの締切は５月４日です。
The application deadline is May 4th.
A period of four days (duration).
夏休みは４日しかないので、短いと感じた。
I only have four days of summer vacation, so it felt short.
展示はあと４日で終わります。
The exhibition ends in four days.
彼は入院してから４日で退院した。
He was discharged four days after being hospitalized.