This is a general counter for four unspecified items or "four pieces." Use this word when you are counting general objects for which there isn't a specialized counter, like "four boxes" or "four problems."

形(かたち)や種類(しゅるい)を問(と)わず、物(もの)を「四(よっ)つ」数(かぞ)えるときに使(つか)います。箱(はこ)四(よっ)つ、問題(もんだい)四(よっ)つなど。