noun
4th day of the month
Refers specifically to the fourth calendar day.
会議は４月４日です。
The meeting is on April 4th.
I stayed in Tokyo for four days.
Explicitly marks a duration of four days, while 四日 can mean either the date or the duration depending on context.
Compound of 四 (yo, 'four') and 日 (ka, 'day'), with a geminate consonant shift yielding よっか.