I spent 6 nights at Carlos' apartment but I didn't meet him - just my host, named Brian. The building is located in front of a square in a residential street with some stores. It is very close to a busy avenue where you can take most buses lines. You can also walk to many tourist attractions.
In my opinion the apartment fits better 2 people and not 4 - 2 people will sleep on a comfortable bed in a room with air conditioner but the 2 other will stay in a sofa bed on the living room. During summer it gets very hot and impossible to sleep with the windows closed - but if you open it the mosquitoes will bother you a lot. This would be easily solved by a fan. Besides this the kitchen supplies could be renewed - the baking pans inside the stove aren't suitable if you really want cook/bake anything.
I strongly recommend trying to book a taxi before your arrival. Taxis are expensive at the Aeroparque airport and I got upset that Brian didn't warn me about this. I also recommend evaluating the check-out time at midday because Brian didn't allow me to leave the bags at the apartment even tough my flight was at 10pm.