Thanks for this educational program.
The correct form is ‘a four-bedroom apartment’.
When NUMBERS and NOUNS are combined to form an adjective, a hyphen (-) is used. Because the new word is an adjective, it is always SINGULAR (so we don’t use ‘s’ at the end).
Some examples:
- She has a ten-year-old grandson.
- We run 3-week courses.
- There’s a two-year guarantee on this mobile.