1 2 3 4 5 6 7 8 9 10 하나 둘 셋 넷 다섯 여섯 일곱 여덟 아홉 열 -These are used when you count how many. ex. 한개 두개 세개 네개 다섯개 여섯개 일곱개 여덟개 아홉개 열개 (개, pcs) 일 이 삼 사 오 륙 칠 팔 구 십 - These are used when you read a certain number. Therefore if you are counting 계절, then it is like 한계절, 두계절, 세계절, 네계절 But if you are just reading a text itself, "4 season" 사 계절 it becomes. In Korea, there are 4 seasons with clear timeline in a year, "4 season" is used as a noun. So we use the word "사계절" often. I hope it helps