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).