why 4 seasons is 사계절, 2 seasons is 두계절?? "사" , "두" different kind of number

Question

why 4 seasons is 사계절, 2 seasons is 두계절?? "사"  ,  "두"  different kind of number

Answer

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