"Three days are not four days."
Translation:사흘은 사일이 아니에요.
There are two equivalent systems for counting days in Korean, a Sino-Korean system and a Native Korean system.
일 일 (1 day), 이 일 (2 days), 삼 일 (3 days), 사 일 (4 days), 오 일 (5 days), etc.
하루 (1 day), 이틀 (2 days), 사흘 (3 days), 나흘 (4 days), 닷새 (5 days), etc.
People commonly use the Native Korean system for 하루 (1 day) and 이틀 (2 days), but after that, you'll probably see the Sino-Korean system being used more often, because it's more practical and easier to scale up, since it follows a neat pattern (Sino-Korean number + 일) and can also be written with Arabic numbers (1일, 2일, 3일).
Just a complement: Neither 흘 nor 일 means "day" by itself, they are just suffixes. If you are not counting days or referring to their timespan, you use the word 날, as in "오늘은 좋은 날 이었어 (아이스 큐브)".
but why in the same sentence the first one is in Native Korean and the second in Sino-Korean?