Refers to the fourth day of a month or a period of four days. This reading 'yokka' is irregular and important to remember. It's commonly used when specifying a date or duration.

月(つき)の四(よっ)日(か)目(め)、または「四(よっ)日(か)間(かん)」のように四(よっ)日(か)間(かん)を数(かぞ)えるときに使(つか)われます。特別(とくべつ)な読(よ)み方(かた)です。