My apartment is located in the city-center of Rotterdam. It is only a few minutes walk to the subway, bus and tram station. The street parallel to the apartment is the Nieuwe Binnenweg, which is cool street with lots of bars and nice restaurants. With two double beds the appartment is perfect for any group of 4.
Please note that the apartment is located at the first floor, you will have to walk 2 sets of stairs. So if you have difficulties walking stairs, I recommend choosing a different place for your stay in Rotterdam:)