This apartment in La Latina was exceptional. It's quite thoughtfully laid out, very comfortable. The 2 Juliet balconies were pretty fantastic; they let in a lot of light and a lovely breeze (it was quite hot when we were in Madrid). But the windows on them blocked sound amazingly well which was occasionally nice as the plaza is quite popular, especially on weekend nights. We had 3 people staying there and there was plenty of space and good privacy. 4 people would do well there, too, I'd imagine. Also we had some friends over for dinner and we found the kitchen to be quite usable. The neighborhood has a bit of edge to it, which we enjoy. Plus you can easily walk to most parts of town (Lavapies, Sol, Chueca, Prado, Retiro), each with their own character. Plus the La Latina metro station is a 5 minute walk, so you can get anywhere in Madrid very easily. Spaniards apparently aren't big on breakfast, but, being Americans, we are. There were 2 really good restaurants with breakfast and pastries close by: L'Adore (owner is French) and Pum Pum. Also, the cappuccino at Cafe del Art, literally across the plaza, was terrific. In addition, two mercados (these large buildings with tons of stalls with excellent meats, seafood, tapas and fruit/vegetables) are within walking distance.
I'd say one of the best features was how responsive Fernando was. We've stayed in a good number of vacation rentals and Fernando was one of the easiest and most responsive to work with.