저는 4시쯤 과 사무실에 있을 거예요. 단, 술만 안 돼겠어요. I will be at the department office around 4 o'clock. But, only alcoholic beverage will not be allowed to drink. 과 means "department" same as 학과 과 사무실 means "department office" 단 means "only, but" same as 다만 단 can be used for emphasizing 한번 한번 means "once" 단 한번 means "only once"

Well actually I can't understand what you wrote. Becuase for me I need more sentence to get what the meaning is. But I wrote a few answers. 저는 4시쯤에 사무실에 있을거에요. -> I'll be in the office around 16:00. We add 단 when we want to emphasise the meaning, for example, I've never ever been there.