People reading these specifications are interested in how many rooms the place has specifically for sleeping. A living-room or lounge should be so-called in your description. If they move in and choose to sleep in the lounge that's their business.
A 4-bedroom flat has 4 bedrooms; we know nothing about how many other rooms it may have. A 4-bedroom flat could be rather grand, and could also have a drawing room, a library, a dining room, a dressing room, and so on.
A 4-room flat has 4 rooms. How they are used is not specified; you could put a bed in each and use all 4 as bedrooms, or you could have only one bedroom, and a living room, a kitchen, and a dining room.