You can create a listing for the specific room and then go to your admin, navigate to the page that shows the calendar for that room and click "Pricing Settings" located in the charcoal menu bar on the right side. Scroll down and click on "extra charges and currency." Tick the box beside "Additional Guests" and you will be able to charge for additional people.
The drawback for you, however, is that once the room is rented by one person, all of the beds will be automatically booked, as it will not distinguish between 1 bed or 4 beds, but only one room. Thus, most people with hostels, I think, advertise by the bed and state in the listing that more beds are available with links to click to view the additional listings.
Good luck!