The perfect family apartment! We just got back from two weeks on rue Beranger, and cannot say enough good things. It's a beautiful, well decorated space and had everything we needed and more. A fourth bedroom had just been added before we arrived, and I'm not sure how that space was used prior, but a nice 4-bedroom in the Marais is hard to come by! Many others have extolled the virtues of the apartment - so I will address two concerns I had before arriving, which were for naught. This place is awesome and if it's available for your dates, grab it!
1) "Republique is a busy, non-charming area. Is the apartment on a nice street?" YES! Rue Beranger is a charming classic Marais street, unlike many of the other streets that feed into Republique. It's super convenient to the Metro on the one end of the street, and on the other end is the best little boulangerie, Tout Autour du Pain. We had the windows open nearly the whole time and we found it relatively quiet. No reservations about the location!
2) "No air conditioning...how did that work out?" The apartment gets great cross ventilation, and with fans we had zero issue. Usually it drops down to high sixties at night. We were there during the 105 degree July heat wave days, and we survived!
And one other note about sleeping arrangements - the bunk bed is extra long and could actually sleep two smaller kids, with heads at opposite ends. The owners were really smart about maximizing space and this bed and the one in the piano bedroom are custom sizes, very cool.
We will 1000% come back to this apartment next time we're in Paris. Vinicius and Diego were both super responsive and helpful over email and WhatsApp. We felt very well cared for and it was an absolute pleasure to live like a local for a couple of weeks. Thank you Vinicius and Diego!