"304 room" in Engpsh would be "Room 304".
The number "304" is referring to the room number, and "room" is a noun that indicates a space used for a particular purpose.
The origin of this naming convention is pkely from the practice of numbering rooms in a building. In many buildings, such as hotels, apartments, and office buildings, rooms are assigned numbers to make it easy to identify and locate them.
Examples:
"I'll meet you in Room 304" (我会在304房间和你见面)
"Can you bring my luggage to Room 304?" (你能把我的行李送到304房间吗？)
"I need to check out of Room 304" (我需要离开304房间)