The flat is in a handy position (well coonected with airport by bus and metro), close to the centre (40 min walking, anyway close to metro stop) and with lots of amenities around (bar, supermarkets).
The space is ok for four people, even though the kitchen is not well equipped to prepare lunch or dinner. On other hands the cofee machine works fine.
The lift is very small, it carries max 1 slim person with cabin luggage for a max of 100Kg. Large luggage or large person might be reach the 3rd floor climbing a narrow stairs.
Overall, I reccomend it for a weekend holidays in Barcelona.
Keep in mind to add over the airbnb bill, the city tax of 2.5 € por person/por day, to pay at the check-in.