Recorded in
《左传·昭公二十六年》:
亦唯伯仲叔季图之 - the four characters mark the four positions of seniority among brothers in classical Chinese naming. You'll often see them embedded in given names:
伯夷 Bóyí (eldest brother Yí),
仲尼 Zhòngní (Confucius, the second son).