We had a great stay at Flora & Luca's place just off Piazza Barberini. The location was very convenient, close to a Metro station, easy to find, lots of restaurants and shops nearby and a short walk to some of sites (e.g. Trevi, Spannish steps). The apartment itself was lovely and true to the pictures; bright, pretty, comfortable and well-equipped. The building was also very secure and the area safe. We were just 3 people staying in the apartment so the size was fine for us I think it would also be fine for 4 people so long as you are close friends. There isn't full separation between the bedroom and living room (where the sofa bed is) - this was fine for us as we were all immediate family but if you were staying in two couples it might not be ideal as it meant you didn't have too much privacy and if one room had their light on it would permeate into the other room, so we had to go to sleep around the same time. Flora & Luca were very helpful; they provided lots of advice by email before our trip, left a map and a booklet with recommendations during our stay and were always available if we called/text them to answer any questions.