We had a very nice stay with our family 3 adults (one young adult). (3rd floor no elevator, i think nowhere in the center) The appartment was well cleaned. A perfect place in the center of Lissabon. And there was even a small heater. We used it carefully just for a few hours to get it warmer (january) In the night and when we went away we did not use it. We did not want to use to much electricity (these heaters use a lot of electricity)
As already mentioned the bed in the living room is small. Bed is 120 cm (made from a couch) i think with a matrass above it, from 140 cm i think. So for one person fine. For a family with 2 young children it as also handy. This because the price stays the same with i think the 3rd and the 4th person.
But if you do not mind you can sleep with 4 if you do not mind a very small bed.