Number + 天
天 (tiān) functions as a measure word for counting days. It is placed after a number to indicate a duration, such as 四天 (four days). It is also used as a noun meaning "sky" or "day" in other contexts.
我请了三天假,打算去海边玩一玩。
Wǒ qǐng le sān tiān jià, dǎsuàn qù hǎibiān wán yi wán.
I took three days off and plan to go to the beach for fun.
这个项目还需要四天才能完成。
Zhège xiàngmù hái xūyào sì tiān cái néng wánchéng.
This project still needs four more days to be completed.
他每天工作十个小时,连着干了五天。
Tā měitiān gōngzuò shí ge xiǎoshí, liánzhe gàn le wǔ tiān.
He worked ten hours a day for five days straight.